Fun Facts

Interesting information about amateur radio.

VHF Propogation Map

The ARRL posted a link to this on their Facebook page.

The map shows activity that has happened in the past hour. Paths are combined to create color-coded footprint indicating the distance VHF signals are likely to be traveling. Packet stations typically run low power into small vertical antennas. Better equipped stations should exceed the the distances…

aprs.mountainlake.k12.mn.us

Tags:

The Birds and The Bees of SWR Knowledge Part 2

http://www.arrl.org/files/file/Technology/tis/info/pdf/q1106037.pdf

     The operative part of this first part of a previous article/reminder I
sent is that you calculate coaxial cable loss by reading the open or short
ended cable's SWR.  If the cable is lossy, power is lost going up to the
antenna, and in a short or open -ended cable all the power that arrives is
reflected back.  Low loss (in dB) going up the (short?, good quality) coax
cable will have low loss coming back as reflected power, ..., and more
reflected power means to be measured at the input end = higher SWR.  In
this case, a high SWR is a good  thing, a high SWR indicating low losses.

     If, on the other hand,
Understanding SWR by Example

Take the mystery and mystique out of standing wave ratio.
Darrin Walraven, K5DVW

From November 2006 QST © ARRL

It sometimes seems that one of
the most mysterious creatures
in the world of Amateur Radio is
standing wave ratio (SWR). I
often hear on-air discussion of guys bragging
about and comparing their SWR numbers
as if it were a contest. There seems
to be a relentless drive to achieve the most
coveted 1:1 SWR at any cost. But why? This
article is written to help explain what SWR
actually is, what makes it bad and when to
worry about it.
What is SWR?
SWR is sometimes called VSWR, for
voltage standing wave ratio, by the technical
folks. Okay, but what does it really mean?
The best way to easily understand SWR is by
example. In the typical ham station setup, a
transmitter is connected to a feed line, which
is then connected to the antenna. When you
key the transmitter, it develops a radio frequency
(RF) voltage on the transmission
line input. The voltage travels down the feed
line to the antenna at the other end and is
called the forward wave. In some cases, part
of the voltage is reflected at the antenna and
propagates back down the line in the reverse
direction toward the transmitter, much like
a voice echoing off a distant cliff. SWR is a
measure of what is happening to the forward
and reverse voltage waveforms and how they
compare in size.
Let’s look at what happens when a transmitter
is connected to 50 Ω coax and a 50 Ω
antenna. For now, pretend that the coax cable
doesn’t have any losses and the transmitter
is producing a 1 W CW signal. If you were
to look at the signal on the output of the
transmitter with an oscilloscope, what you
would see is a sine wave. The amplitude of
the sine wave would be related to how much
power the transmitter is producing. A larger
amplitude waves means more power. This
wave of energy travels down the transmission
line and reaches the antenna. If the antenna
impedance is 50 Ω, just like the cable, then
all of the energy is transferred to the antenna
system to be radiated. Anywhere on the
transmission line you measured, the voltage
waveform would measure exactly the same
as the sine wave coming from the transmitter.
This is called a matched condition and is what

Take the mystery and mystique out of standing wave ratio.
Darrin Walraven, K5DVW
Table 1
SWR vs Reflected Voltage or Power

VSWR            Voltage                 Power
                Reflected (%)   Reflected (%)
1.0:1           0               0
1.1:1           5               0.2
1.2:1           9               0.8
1.3:1           13              1.7
1.4:1           17              2.8
1.5:1           20              4
1.6:1           23              5.3
1.7:1           26              6.7
1.8:1           29              8.2
1.9:1           31              9.6
2.0:1           33              11
2.5:1           43              18.4
3.0:1           50              25
4.0:1           56              36
5.0:1           67              44.4
10.0:1          82              67

     So, a high SWR into your open-ended (new or old) coaxial cable
indicates most or "all the power reflected" is making it back to the SWR
meter to be read has a high SWR, which means low loss.  If very little
reflected power (or none of it) makes it back to the SWR meter, it reads a
low SWR, but means the cable is very lossy.  Tomorrow, I'll find the "SWR
in dB Loss" chart and give the exact values.

6 and 10

Attached is a possible multi-band antenna from the MARCH 2015 issue of
QST  to put up in the attic to be able to get yourself on 10 and 6 meters
quickly and easily to try them out.  Uses the two conductors of 450 ladder
line to make a dual band antenna, such as 10 m and 6 m "in one flexible,
cheap, unit."  Doc's (W8NRH) sea stories suggest 6 m SSB (SSB is typically
horizontal  polarization, FM is typically vertical  polarization for mobile
work, BUT, contacts CAN be made both ways) can open up and be worked on a
'wet noodle' of an antenna!

     Several of us have quad-band FM radios and have been operating on 6 m
FM (52.700 Repeater in Columbus, -1.0 MHz input site is closest to
Marysville, 123.0 Hz, see http://www.corc.us/).  If I make my antenna, I
might also take check-ins on a future NET on 10 m FM (29.600 MHz simplex,
but not for Novice/Techs) and / or 6 m FM (52.525 MHz simplex, for all
except Novices) just to try something different!  (Frequency privileges:
http://www.arrl.org/frequency-allocations )   Could even do 10 m SSB phone
(28.300 to 28.500 MHz) to include any Techs and Novices!  (Any Novices left
around here?)  The above ladder line dual band antenna could be a 10 m / 6
m dual band horizontal dipole for SSB if just the horizontal 'long
radial' (even the folded version) was made, then cut and fed in the middle
of the 10 m length like any standard dipole!

(See attached file: Close Coupled Resonator multi-band ant.doc)

Early FM Mobile Radio Tests

Here is some of the developmental history from 1941 of how FM as we
know it today got started and tested by GE, with FCC Engineers in tow. FM
was only invented 1935 by Major (name, not rank) Edward Armstrong. Of note
is that these FM mobile radio tests started at +15 kHz FM deviation and on
40 kHz channel spacing! That means it was only re-farmed once in the mid
1960's to what was called "Narrowband" of +5 kHz FM deviation and to 20
kHz channel spacing on VHF Low-Band (30-50 MHz), which was called "UHF"
back then. On VHF High Band (138*-150-174 MHz) it became 15 kHz channel
spacing as the filters became better by the time 150 MHz became available
(after WW II). I had always thought there were two re-farms to get down to
+ 5 kHz deviation, starting at + 30 kHz.

 

Any way, here is the article that describes the history of the Tests
by GE that set FM as better than AM. Enjoy!

 

http://www.americanradiohistory.com/Archive-Communications-Magazine/1940s/Communications-1942-02.pdf

see magazine page 5, pdf page 7. If it only goes to the Home page, select February 1942.

 

In 1995, the FCC announced the "Land Mobile" communications bands
(Police, Fire, Business, TRC, etc.) radios would be re-farmed again to +
2.5 kHz FM deviation = 100% modulation on 7.5 kHz channel spacing in 2000.
Well, it took until 2013 to effect all the changes necessary! And the plan
to re-farm again in short order to + 1.25 kHz FM Deviation on 3.0 kHz
channels was in that same plan for the future (2020??).

 

Where AM has natural electronic limits to 100% modulation (where the
waveform "crosses 0" on the negative-going waveforms, FM does not.

FM "100% modulation" is a man-made construct. For
different services, (Land Mobile, FM broadcast radio, TV audio) can and did
have different "100% modulation" limits to the deviation amounts.

 

http://wireless.fcc.gov/services/index.htm?job=service_bandplan&id=industrial_business  ,

graphic at    http://wireless.fcc.gov/services/plmrs/images/vhfband.jpg

or https://en.wikipedia.org/wiki/Business_band

* The Federal Government equivalent of the FCC is the NTIA (National
Telecommunications and Information Agency for the FBI, Military, Secret
Service, Postal Inspectors, etc.) They have their own bands around the FCC
bands for the same purposes (Land Mobile two-way radios). The NTIA and the
FCC often move together on such issues as radio specs so manufacturers can
make radios to common and equivalent specs.

FCC Bites in OH; EMA Notes

First, just when you think there is no FCC Enforcement action and/or
bite:
from  https://transition.fcc.gov/eb/Orders/2015/DA-15-932A1.html
******************************

***********************************
Before the Federal Communications Commission, Washington, D.C. 20554
In the Matter of Daniel R. Hicks,  Licensee of Amateur Radio Station
KB8UYZ, Cincinnati, Ohio
* NOTICE OF APPARENT LIABILITY FOR FORFEITURE

Adopted: August 20, 2015 Released: August 20, 2015

By the District Director, Detroit Office, Northeast Region, Enforcement
Bureau:

INTRODUCTION
* We propose a penalty of $8,000 against Daniel R. Hicks for intentionally
causing interference to other amateur radio operators and failing to
provide his proper station identification. ...In addition, the failure to
transmit a licensee's assigned call sign information disrupts the orderly
administration of the Amateur Radio Service by preventing licensed users
from identifying a transmission's source.

* In this Notice of Apparent Liability for Forfeiture (NAL), we find that
Mr. Hicks, licensee of Amateur Radio Station KB8UYZ in Cincinnati, Ohio,
apparently willfully violations Section 333 of the Communications Act of
1934, as amended (Act), and Sections 97.101(d) and 97.119(a) of the
Commission's rules (Rules) by causing intentional interference to licensed
radio operations and failing to transmit his assigned call sign in the
Amateur Radio Service. ...

* On March 3, 2015, in response to continued complaints of interference, an
agent from the Detroit Office returned to the Cincinnati area to again
attempt to identify the source of the transmissions. This time the agent
did not advise the local amateur radio group that he was in the area. The
agent used mobile direction finding techniques to locate the source of the
transmissions to 4472 Forest Trail, Cincinnati, Ohio, the address of record
for Mr. Hicks' amateur station KB8UYZ. ...
...
ORDERING CLAUSES

* Accordingly, IT IS ORDERED that, pursuant to Section 503(b) of the Act
and Sections 0.111, 0.204, 0.311, 0.314, and 1.80 of the Rules, Daniel R.
Hicks is hereby NOTIFIED of this APPARENT LIABILITY FOR A FORFEITURE in the
amount of eight thousand dollars ($8,000) for willful and repeated
violations of Section 333 of the Act and Sections 97.101(d), and 97.119(a)
of the Rules.

* IT IS FURTHER ORDERED that, pursuant to Section 1.80 of the Rules, within
thirty (30) calendar days of the release date of this Notice of Apparent
Liability for Forfeiture, Daniel R. Hicks SHALL PAY the full amount of the
proposed forfeiture ....

[There was more I can't find right now that basically said Hicks filed a
rebuttal that said 'You can't prove it was me,' to which the FCC said 'Yes,
we can, and, yes, we did.  Since your appeal showed no reason why you can't
pay the full fine, pay the full amount in 30 days.  So ordered.']

      Second, some after-action reports from events just after Katrina hit:

     The below excerpt from the article (link) is with the Chief Engineer
at the only New Orleans AM radio station that stayed on the air (well, all
six that they owned and operated) after Katrina hit.  The whole article is
a good read, but this part in particular struck me as a real potential
problem.  If we, as Operators, are going to operate, 1) we need to know our
families are safe and safely out of the area, and 2) the unexpected influx
of "just one or two" or more from everybody's family quickly overwhelms the
planning for the sustenance of the Operators.  Now is the time to think
about this, ahead of time, and make plans for where and when  your family
will go.  Pets are a particular problem, especially for >24 hours, let
alone 30 days.  The bold is mine, but what caught my eye.  The whole
article is a good read.

http://www.radioworld.com/article/special-report-when-disaster-hit-they-kept-wwl-on-the-air/276946

(and
http://www.radioworld.com/article/katrina-put-crisis-plans-to-the-test/16711
 )

RW: You’ve talked since then at more trade shows about lessons learned
regarding communications, emergency plans, generators, fuel and so forth.
What are the most important lessons for radio managers?

Pollet: Our biggest surprise, one day prior to Katrina’s arrival, was the
unexpectedly large number of staff members who showed up at the studios
offering to help in any way possible. Many, if not most, brought along a
spouse, kids, parents and even dogs and cats. In reality, many were just
seeking shelter from the storm without having to drive hundreds of miles in
the mandatory evacuation process that was in effect. Many would probably
later regret not evacuating when they still had the chance. This unexpected
influx was obviously going to tax our small food supply far beyond its
limit. In fact, the large number, estimated at more than 50 additional
people, was probably the primary reason we were forced to evacuate what was
an otherwise viable site several days after the storm.

We now have a strict emergency event participation policy in place.
Staffers who agree in advance to stay during an emergency event must make
other arrangements for the safety of family and pets. We’ve also learned
that our previous emergency supply plans were woefully inadequate. Prior
hurricanes were always one- or two-day events. Our supplies were based on
that time frame and consisted of little more than a few loaves of bread, an
assortment of cold cuts and a few bags of assorted snacks. We now have
enough MRE-style food on hand to last for at least one month. Drinking
water is stored in advance as are water purification supplies. We slept in
chairs or on the floor after Katrina. We now also have a large supply of
air mattresses, portable showers and personal hygiene items on hand all
sufficient for an event lasting at least one month. - See more at:

And, after action analysis:

Number 2 would have been having a security presence, and additional food
supplies, flown in by helicopter after Katrina passed instead of having the
air staff and engineers flown out. In effect, we wound up abandoning what,
at the time, was a working viable downtown studio site [ACC: due to food
and security issues for the Operators, not because they were without power
(natural gas generator) or failure of the equipment.]

The Birds and The Bees of SWR Knowledge

So much good discussion about SWR (Standing Wave Ratio) lately on the
Repeater, and to get some new Hams off to a good start on the right foot,
here are some facts your parents should have told you (presuming they were
Hams and knew better themselves when they were kids....).

     SWR is one of those mythological creatures that has taken on a Medusa
and Pandora quality of its own.  So much  mis-information, innuendo, facts
that just aren't so, and multiple wrong-headed ideas are out there in
Hamdom, and so much of it is half truths, or outright wrong!
Unfortunately, we can't put all that back in Pandora's box.  The link below
is a great article from the ARRL in QST that I wanted to get everyone new
off to a great start on reading the facts of RF Life and Line Reflections
about SWR.

     Below is a snippet, particularly the boldened part by me.  Most people
will know that a non 1.0:1 SWR will mean some power is being reflected from
the antenna (not radiating all the power out because of an impedance
mismatch) back down the transmission line.  But few seem to know that the
antenna-reflected power arriving back at the transmitter is re-reflected
back up the coax again, most of which (% by SWR at the antenna) will be
radiated when it returns to the antenna.

     That is a little-acknowledged fact.  From there, things only get
deeper and more algebra/ trigonometry/ calculus driven, such as the phase
angle of the reflected voltage, etc.  (If you want a good dose of headache,
read the part about Smith Charts in the article.  "The concept behind the
Smith Chart is simple.")  But, if you will remember that "*all*" (wink,
wink, nod, nod) the power is radiated, regardless of the SWR, then you will
be one step ahead of the less-knowledgeable crowd.  But wait, there is
(unfortunately) more!!

http://www.arrl.org/files/file/Technology/tis/info/pdf/q1106037.pdf

Does Higher SWR Lead to Lower
Power Being Transmitted?
Not always so dramatically. Believe it or
not, 100 percent of the power is actually transmitted
in both of the previous examples. In the
first case, with a 50 Ω antenna, it’s easy to see
how all the power is transferred to the antenna
to be radiated since there are no reflections.
In the second case, the 33 percent voltage
reflection travels back down to the transmitter
where it doesn’t stop but is re-reflected from
the transmitter back toward the antenna along
with the forward wave. The energy bounces
back and forth inside the cable until it’s all
radiated by the antenna for a lossless transmission
line. An important point to realize
is that with extremely low loss transmission
line, no matter what the SWR, most of the
power can get delivered to the antenna. A later
example will show how this can happen.

     *OK, the all-important caveat.  I said "*all*" the power is radiated
regardless of the SWR.  The * fine print * is, "all the RF power that is
not lost in the transmission line."  As the article points out, that is an
important caveat.  I have three cases of "loss in the transmission line" to
consider.  Please DO read the article.  These three cases are my own
examples.

     First, unless you 1) either are measuring SWR right at the antenna
feed point, or 2) NEED to measure it accurately for design reasons, SWR is
an indicator of efficiency and approximate "quality factor" of your antenna
system,  but it is hardly absolute without extreme measures taken to make
it  absolute.

1)  Typical coax has loss, no matter what frequency, no matter how long, no
matter what size.  It is in every factory spec.  It is that loss for your
length of coax, type of coax, and band you are operating on  that compounds
the Reflected Power Loss = SWR issue.  Fine, your HF wire antenna used on 6
m shows you in the shack at the transmitter end  that you have a 2.0:1 SWR
(11% Reflected Power from article Table 1). ...  BUT, that could be 5.0:1
AT THE ANTENNA if your cable has 3.0 dB (1/2) power loss at VHF (compared
to the < 2.0 dB of loss at HF you bought it for)!

     Of your 100 W Shack-end SWR meter-measured output power, for 3 dB coax
loss (1/[10^[3 dB/10]]), only 50 W is reaching the antenna through the 3 dB
(half power loss at VHF, not at all uncommon, and more common by UHF) coax
loss.  Of the 50 Watts reaching the antenna, at a 5.0:1 SWR, 44.4% is being
reflected (Table 1) = only 55.6% (only 27.8 W) is being radiated.  [50
Watts of RF lost in the cable (its 3.0 dB loss = 1/2 =) 50 Watts of heating
the coax!  No different than putting it an oven...]

     For a 5.0:1 SWR AT THE ANTENNA (e.g., bad antenna, broken balun, not
meant for that band, operating way outside of its tuned "2.0:1 SWR" points,
etc.) (Table 1 says) 44.4% of the power arriving is being reflected back
down the coax, = 22.2 Watts.  On its way back down that same 3 dB loss coax
cable, the REFLECTED power also suffers a 3 dB loss in the coax on the
Return trip, such that, 11.1 more Watts heats the coax (now up to 61 Watts
lost in the coax!!!), AND only 11.1 Watts reaches the SWR meter to be
measured as Reflected Power for SWR display in the Shack!  That "11%
Reflected Power" (from Table 1) of the direct value measured going into the
coax from the transmitter means that your Shack SWR meter will show you a
2.0:1 SWR!!

     Hence, not only does better coax (and good connectors) get more power
(from less loss) TO your antenna, it also helps you see 'closer to the
truth' of what RF power is being reflected from the antenna = coming back
down the coax with less loss to the reflected power for a more accurate,
but never-really-quite-correct, SWR value in the Shack.

     Rerun the above example with 1.5 dB (1 - 0.708 factor) of coax loss
(e.g., larger diameter cable, fix/replace a bad connector (PL-259 @ UHF)
with 2.0 dB of loss at UHF!! with an N connector), and the 5.0:1 SWR at the
antenna (no change to how the antenna was really working!) gets displayed
as 3.0:1 SWR, but, with even MORE POWER OUT THE ANTENNA (70 Watts to the
antenna x [1- 44.4% reflected]) = 39 Watts radiated), in spite of a higher,
but more accurately reported/meter reading SWR of 3.0:1  !!!! !

     And that folks is how SWR is a double-ended, twice-twisted, inside
out, enigma wrapped inside a riddle.

2)  And now you can understand why some blue-blooded Hams (and commercial
circuits, military) go to using (make their own) air dielectric, open bay,
ladder line.  Losses of 0.2 dB per 100 feet at HF are possible! ...  But
that stuff has lots of mechanical issues and precautions to overcome that
coax does not have.

     So now, with ladder line, even if the SWR at the antenna feed point is
5.0:1 or higher, who cares if the reflected power suffers a 0.2 dB loss
(<5%) loss, coming back down, it only suffers that same loss going back up
to the antenna again, where 55.6% of it is radiated again, ....  So, with
very low loss feed lines (not just coax), SWR plays even less importance to
antenna system efficiency.  However,  there are real impacts of a 10:1
VOLTAGE RATIO (VSWR) on the line for a 12 V transistor made to handle only
50 V!  You get the idea, .....  For 100 W into 50 ohms, it is 70.7 V RMS =
~ 100 peak voltage x 10:1 ratio = 1000 V on the line = trouble = arcing and
sparking for many circuits designed with 12V (to 50 V) in mind, not 1000
V!!!  And you thought 500 V capacitors were over-kill!!  THIS is one good,
solid, reason to keep SWR down.  They didn't (know or) care about SWR in
the early days of tubes!

3)  What about loss-less feed line lengths?  How long is the "feed line"
inside your handheld radio to the rubber duck ON your handheld??
Essentially, the short length of connector wire to the antenna socket
inside a handheld radio is part of the RF power transistor + antenna filter
+ impedance matching circuit.  You "tune" the (handheld) transmitter for
the (correct) load attached to it, typically 50 ohms, so the test meter
readings are reasonably accurate.  Even if the rubber duck is 5 or 500
ohms, with essentially "no feed line" to speak of, "all the power gets
radiated."  While not entirely correct to the design engineer (which I was,
so I get lost in/concerned over this minutia), it explains how, as bad as
rubber ducks are, how the do seem to 'work' in some fashion after all.  A
3:1 SWR or more is quite typical for a rubber duck.  Yet, they work, (even
though I say they are  -10 (1/10) to -20 dB ((=1/10^[20/10]) = 1/100 = 1%!)
efficient)!!

     All this being said, coax and connectors never get "better" with age.
Coax cables and connectors, (and baluns, line isolators, wires,
connections. etc.) are always "best when new."  So, record the numbers
(measured coax loss before you put it "up" out of reach, SWR, etc.)  when
put up new, and date it!  Then, if/when the SWR gets better  at some time
in the future on its own, you can bet your bottom dollar the loss in the
coax has increased, certainly not decreased on its own.  (Water in the
coax, something broke, etc.)  We come back to more coax loss reads out as a
better SWR when it really shouldn't.  10 dB of coax loss would be only 1%
Reflected Power (up and back) measured in the shack = 1.2:1 SWR from a
completely open-ended or shorted-at-end coax with no antenna attached!

     In fact, this is one way to check new coax cable, open-ended or
shorted, if low loss, and "all" the test power into the coax gets reflected
from the open or shorted end (and it all should be), then a (very) high SWR
reading means the Reflected power is NOT getting lost in the coax.  The
higher the SWR for open (or short) ended coax cables, the better (lower
loss) the coax is!  A 2.0:1 SWR into 100 feet of RG-8X tested (-4.5
dB/100', up and back = -9 dB) at 146 MHz is bad news for VHF.
http://www.universal-radio.com/catalog/cable/coaxperf.html   And that is
just the loss-to-reflected-power that has NOTHING to do with the antenna
radiating efficiency (or not!).  Presuming the antenna IS radiating some
power, the SWR readout will be even lower.  A true 5.5:1 SWR for the
antenna proper would make this example's readout in the Shack about a 1.6:1
SWR.  Everything is fine, right??????

Line A

Do you know the infamous FCC's "Line A" does run through the northern
tier of Ohio?  What does it mean to you and your operations on 420-450 MHz?
Even though Union County is not within Line A, there are other issues and
(such as Commercial Licenses by the FCC in the 420-430 MHz sub-band, and
Canadian Primary status in the 420-430 AND 440-450 MHz sub-bands) that we
must "not cause interference to."  Just a little known, little used set of
rules that could impact your operating in Ohio that we need to keep in
mind.  Scott N8SY started the conversation and article, I emailed him some
specifics that he saw fit to include....

http://arrl-ohio.org/     Something you might want to add to your
Favorites.

http://n8sy2.blogspot.com/2015/06/june-2015-edition-of-ohio-section.html
Go to the very bottom right of that page (above) and click on the "Line A -
What's it all about" link if the below only takes you to the main page:

http://arrl-ohio.org/line_a/line_a.html

FM, The New Fangled Fad

And a reminder that what we use or are so accustomed to today isn't even 100 years old yet...

 Today in Radio History:

 June 14, 1922 WEAR, Baltimore, broadcasts President Harding's speech at the dedication of the

Francis Scott Key Memorial, the first time a U.S. president has been broadcast live.
 ...

 June 16, 1934 Armstrong transmits FM signal 70 miles from Empire State Building to Long Island

     To contrast that, the United States Navy Reserve (USNR) is 100 years old in 2015.  I spent 28 years in the USNR +1.5 years augmented to USN, including a total of 15 years on active duty.  I go back to Indianapolis after the Hams & Egg Breakfast, put the uniform on again, and have some pictures taken with two other shipmates, one another Commander (female Naval Academy Grad).  And Popeye - in every sense of the word, looks, and gruff.  I was a (full) Commander (O5).  O5s are to be Retired by law "upon completion of 28 years of Commissioned Service."  Recalled to Active Duty in 2006, I was extended on Active Duty for one year beyond my mandatory retirement date - virtually unheard of in the Navy.  I have 29.5 years of Commissioned Service, she has 33 yrs, both for special reasons.  The uniqueness of this is that just three of us, two Commanders + a First Class Petty Officer total 100 Years of Service between the three of us, as much (ahem, as old) as the Navy Reserve is!  Not Admirals (limited by the President), or even Captains (who could do a maximum of 30 years), but two Commanders and a (rare, not yet) 40-years of Service Navy sailor.

Point is, "FM radio" is still that 'new-fangled fad (at still less than 100 years old) that will never catch on...', and three of us have a combined Service older than FM radio!!         

See ya at Breakfast!

Arnal Cook

Some Coaxial Cable Facts and Trivia

A little data from Wiki.  The 1/4 wave antenna impedance is ~ 36 ohms
resistive (at resonance), and a 1/2 wave dipole cut-and-fed in the middle
(1/4 wave to each side) as a nominal 73 ohms of Radiation Resistance (left
as the "impedance" where XL and XC cancel each other out), where as the 1/2
wavelength antenna, fed at the end, is a very high (>3,000 ohms) impedance
antenna and seldom used for that reason (exception:  thru-the-glass
antennas and their special and unique conditions*).  From there, I have
mentioned about why we use 50 ohm coax, and how happily it makes a good
match to which ever 1/4 wave (mobile) or 1/2 wave (center fed) dipole
antenna we happen to be using without having to keep two kinds of coax (and
never enough of the 'right one' we need more of) around!  30 ohms handles
the most power for its size, and 76 ohm has the least loss for its size.
Below from Wiki are some of those facts from a source other than off the
top of my head.  Enjoy!

     *Feeding an end-fed antenna can be very difficult, and even more so
when the end-fed is indeed 1/2 wavelength long.  The end point is a very
high impedance, between 1,000 and 10,000 ohms.  The end of a 1/2 wavelength
whip is attaches to a 'plate' which is glued to the outside of the glass.
The inside coupler has a coil and a capacitor in parallel, which makes a
high impedance circuit to feed 'the third plate' on the outside of the
glass in this "high impedance, voltage feed, low current" end of the 1/2
wave whip, through the glass, which itself makes for a great capacitor
insulator - provided it does not have passivating or tinting in it which
contains metal, in which case, all bets are off!.  On the inside, the 50
ohm coax feeds at tap on the parallel coil perhaps only a turn or two up
from where the shield connects 'at the bottom of the coil.'

     Read the following about coax impedances for fun and pleasure.  Did I
say there would be a quiz Saturday morning?

     They did not mention in the text, but did in the notes on the tables
(not cut 'n pasted here, see link) that RG-62 cable at 93 ohms is what is
used in car radios for the AM Band:  "cable has the lowest capacitance per
unit-length when compared to other coaxial cables of similar size.
Capacitance is the enemy of "...among other things, loading a very short
whip antenna for the hundreds of meters long wavelengths on the AM Band
(31" = 1/4 wave at FM Band).  That very low capacitance cable (and a short
length of it at that) helps to keep from "loading down" ("killing") the AM
Band signals in the coax from such a short "voltage probe" of an antenna on
the AM Band.  Since the antenna system is more critical to performance on
the AM Band, and the FM Band signal reception at least gets a much better
start with a full 1/4 wave, 31" tall, antenna, they use the RG-62 to help
the AM Band performance most.  The low capacitance doesn't hurt the FM Band
(much).

     First question:  What is the "geometric mean" impedance of the factors
cited below?  Hint:  Why do we use what impedance cable for radio
transmission systems today?

     What is this all about:  "Installations which need exact matching will
use some kind of matching circuit at the base of the antenna,"?  Hint:
That little metal cap on a Motorola NMO mount 1/4 wave whip.... or an
automatic antenna tuner in my trunk!

http://en.wikipedia.org/wiki/Coaxial_cable

Choice of impedance

The best coaxial cable impedances in high-power, high-voltage, and
low-attenuation applications were experimentally determined at Bell
Laboratories in 1929 to be 30, 60, and 77 Ω, respectively. For a coaxial
cable with air dielectric and a shield of a given inner diameter, the
attenuation is minimized by choosing the diameter of the inner conductor to
give a characteristic impedance of 76.7 Ω.[12] When more common dielectrics
are considered, the best-loss impedance drops down to a value between
52–64 Ω. Maximum power handling is achieved at 30 Ω.[13]

The approximate impedance required to match a centre-fed dipole antenna in
free space (i.e., a dipole without ground reflections) is 73 Ω, so 75 Ω
coax was commonly used for connecting shortwave antennas to receivers.
These typically involve such low levels of RF power that power-handling and
high-voltage breakdown characteristics are unimportant when compared to
attenuation. Likewise with CATV, although many broadcast TV installations
and CATV headends use 300 Ω folded dipole antennas to receive off-the-air
signals, 75 Ω coax makes a convenient 4:1 balun transformer for these as
well as possessing low attenuation.

The arithmetic mean between 30 Ω and 77 Ω is 53.5 Ω; the geometric mean is
48 Ω. The selection of 50 Ω as a compromise between power-handling
capability and attenuation is in general cited as the reason for the
number.[14] 50 Ω also works out tolerably well because it corresponds
approximately to the drive impedance (ideally 36 ohms) of a quarter-wave
monopole, mounted on a less than optimum ground plane such as a vehicle
roof. The match is better at low frequencies, such as for CB Radio around
27 MHz, where the roof dimensions are much less than a quarter wavelength,
and relatively poor at higher frequencies, VHF and UHF, where the roof
dimensions may be several wavelengths. The match is at best poor, because
the antenna drive impedance, due to the imperfect ground plane, is reactive
rather than purely resistive, and so a 36 ohm coaxial cable would not match
properly either. Installations which need exact matching will use some kind
of matching circuit at the base of the antenna, or elsewhere, in
conjunction with a carefully chosen (in terms of wavelength) length of
coaxial, such that a proper match is achieved, which will be only over a
fairly narrow frequency range.

RG-62 is a 93 Ω coaxial cable originally used in mainframe computer
networks in the 1970s and early 1980s (it was the cable used to connect IBM
3270 terminals to IBM 3274/3174 terminal cluster controllers). Later, some
manufacturers of LAN equipment, such as Datapoint for ARCNET, adopted RG-62
as their coaxial cable standard. The cable has the lowest capacitance per
unit-length when compared to other coaxial cables of similar size.
Capacitance is the enemy of square-wave data transmission (in particular,
it slows down edge transitions), and this is a much more important factor
for baseband digital data transmission than power handling or attenuation.

 

Lightning Rod Facts vs Myths Information

Lightning rods attempt to discharge the voltage build-up between earth
and sky/cloud before the flash-over potential gets reached causing
lightning.  How fast one or several pointed lightning rods can accomplish
that movement of a mass of electrons from earth into the sky to reduce the
voltage differential (think of it like discharging a capacitor - a little
bit over time through a resistor, or "all at once" through 0.1 ohms) is the
issue to leave not-enough voltage differential to achieve the catastrophic
flash-over we call lightning.  Large (6 AWG) conductors allow for more
current to flow, discharging the high voltage quicker  (di/dt) reducing the
likelihood of getting hit by making the top of your house or tower static
voltage look like every other blade of grass in the yard, reducing the
approximate 10 kV/m rise in natural voltage above the earth.  Yea, the top
of your 20 meter (~60') tower could be sitting there with 600 kV of
earth-polarity, lightning attracting, static voltage on it without a way to
drain it off, quickly!  Read on

(See attached file: Lightning rod facts vs myths.doc)

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