Jeff wrote:

If there is a mismatch at the antenna (and there is no matching at the
antenna), then maximum power transfer will occur when the conjugate
match is applied at the transmitter end of the feedline.

Surely a conjugate match will only match the load if the coax length is
1/2 wavelength or multiple thereof, and the feeder is also lossless.

Any other coax length will introduce a phase shift that will require a
different match.

Jeff

You just have to match whatever impedance the aerial impedance has been
transformed to at the transmitter end. Then you will get maximum power
into the radiation resistance of the aerial (less the second order
losses in the feeder). A remaining reactive mismatch between the feeder
and the aerial will result in increased voltages and currents and
increased feeder loss (a second order effect at HF) but will not prevent
substantially full power transfer. We had this discussion about very
short aerials quite recently, You have to have a very extreme radiation
resistance for this not to work. Choosing a length of aerial with no
extreme values on the bands you are using is where we came in.

--
Roger Hayter

On 7/1/2015 12:26 PM, Jeff Liebermann wrote:
On Tue, 30 Jun 2015 15:13:55 -0400, Jerry Stuckle
wrote:

Yes, it's most effective to match the feedline to the antenna at the
antenna connection. But it's also important to match the transmitter to
the feedline.

This latter piece is often ignored because people will use a feedline
who's characteristic impedance matches the transmitter already (i.e. 50
ohm line for a 50 ohm transmitter).

However, there are exceptions. For instance, if you're feeding a 75 ohm
antenna (i.e. a dipole) with 75 ohm coax, a 1:1 balun at the antenna
will provide a good match (ideally, 1:1). But there will be a 1.5:1
mismatch to a 50 ohm transmitter. In this case it would be better to
have the matching network at the transmitter.

We may have had this discussion before. Matching a 75 ohm load to a
50 ohm source might be academically interesting, but the actual loss
is almost negligible. for a VSWR of 1.5, the return loss is 14dB and
the load mismatch attenuation is 0.177dB. That's about what I would
expect to lose in two coax connector pairs.

You could also feed the antenna with 50 ohm feedline and place the
matching network at the antenna. The effect would still be a 1:1 SWR,
but the lower impedance of the coax would create higher i^2R losses; not
important if you're talking a short line, but a longer one would lower
output at the antenna.

True, but for roughly equivalent sizes of coax cables, the 75 ohm
cable has less loss and the equivalent 50 ohm cable. If you want to
handle high power, use 50 ohms. If you want low loss, use 75 ohms:
http://www.belden.com/blog/broadcastav/50-Ohms-The-Forgotten-Impedance.cfm
Note that these are for air dielectric cables.

Things are not so neat if we consider the dielectric. See the bottom
paragraph and graphs:
http://www.microwaves101.com/encyclopedias/why-fifty-ohms
Dielectric Dielectric const Minimum loss impedance
solid PTFE 2.2 50 ohms
foam PTFE 1.43 60
air 1.0 75
RG-6/u CATV 75 ohm foam coax still has slightly less loss than the
equivalent 50 ohm cable, but not as much as I've previously claimed.

This is cute:
http://cablesondemandblog.com/wordpress1/2014/03/06/whats-the-difference-between-50-ohm-and-75-ohm-coaxial-cable/
"A good rule of thumb is that if the device being connected
via coaxial cable is a receiver of some kind, 75 Ohm Coax is ideal."



Jeff, do you always miss the forest for the trees? That was an EXAMPLE.
The same would be true if you were feeding a 300 ohm yagi with 300 ohm
twinlead and a transmitter with a 10 ohm output impedance.

And BTW - when calculating, you forgot about the transmitters which cut
back power to protect the finals. Many will do so even with a 1.5:1 SWR.

--
==================
Remove the "x" from my email address
Jerry, AI0K

==================

On 7/1/2015 4:23 PM, Jerry Stuckle wrote:
On 7/1/2015 12:26 PM, Jeff Liebermann wrote:
On Tue, 30 Jun 2015 15:13:55 -0400, Jerry Stuckle
wrote:

Yes, it's most effective to match the feedline to the antenna at the
antenna connection. But it's also important to match the transmitter to
the feedline.

This latter piece is often ignored because people will use a feedline
who's characteristic impedance matches the transmitter already (i.e. 50
ohm line for a 50 ohm transmitter).

However, there are exceptions. For instance, if you're feeding a 75 ohm
antenna (i.e. a dipole) with 75 ohm coax, a 1:1 balun at the antenna
will provide a good match (ideally, 1:1). But there will be a 1.5:1
mismatch to a 50 ohm transmitter. In this case it would be better to
have the matching network at the transmitter.

We may have had this discussion before. Matching a 75 ohm load to a
50 ohm source might be academically interesting, but the actual loss
is almost negligible. for a VSWR of 1.5, the return loss is 14dB and
the load mismatch attenuation is 0.177dB. That's about what I would
expect to lose in two coax connector pairs.

You could also feed the antenna with 50 ohm feedline and place the
matching network at the antenna. The effect would still be a 1:1 SWR,
but the lower impedance of the coax would create higher i^2R losses; not
important if you're talking a short line, but a longer one would lower
output at the antenna.

True, but for roughly equivalent sizes of coax cables, the 75 ohm
cable has less loss and the equivalent 50 ohm cable. If you want to
handle high power, use 50 ohms. If you want low loss, use 75 ohms:
http://www.belden.com/blog/broadcastav/50-Ohms-The-Forgotten-Impedance.cfm
Note that these are for air dielectric cables.

Things are not so neat if we consider the dielectric. See the bottom
paragraph and graphs:
http://www.microwaves101.com/encyclopedias/why-fifty-ohms
Dielectric Dielectric const Minimum loss impedance
solid PTFE 2.2 50 ohms
foam PTFE 1.43 60
air 1.0 75
RG-6/u CATV 75 ohm foam coax still has slightly less loss than the
equivalent 50 ohm cable, but not as much as I've previously claimed.

This is cute:
http://cablesondemandblog.com/wordpress1/2014/03/06/whats-the-difference-between-50-ohm-and-75-ohm-coaxial-cable/
"A good rule of thumb is that if the device being connected
via coaxial cable is a receiver of some kind, 75 Ohm Coax is ideal."



Jeff, do you always miss the forest for the trees? That was an EXAMPLE.
The same would be true if you were feeding a 300 ohm yagi with 300 ohm
twinlead and a transmitter with a 10 ohm output impedance.

Transmitter output impedance does not determine SWR.

And BTW - when calculating, you forgot about the transmitters which cut
back power to protect the finals. Many will do so even with a 1.5:1 SWR.

On Wed, 01 Jul 2015 17:23:54 -0400, Jerry Stuckle
wrote:

Jeff, do you always miss the forest for the trees? That was an EXAMPLE.

Perhaps you missed my point. I don't care about VSWR as long as the
system is reasonably efficient, does not protect itself, and produces
adequate TX power. In my world, that means I'm concerned about
losses, not VSWR. The losses involved in using 75 ohm feedline and
coax in a 50 ohm system are negligible.

The same would be true if you were feeding a 300 ohm yagi with 300 ohm
twinlead and a transmitter with a 10 ohm output impedance.

One of my favorite methods of argumentation is to provide a ridiculous
and extreme example, and then use it as the basis for discussion. I
guess it's a form of "straw man" argument, where I'm now expected to
defend my point of view against your ridiculous and extreme example.
Please forgive me for not following your lead and continuing to be
more concerned with the 50/75 ohm problem:
https://en.wikipedia.org/wiki/Straw_man

And BTW - when calculating, you forgot about the transmitters which cut
back power to protect the finals. Many will do so even with a 1.5:1 SWR.

Could you provide me with one example of such a transmitter? I've
seen such radios on the bench, but they're usually mistuned or
misadjusted.

I did some Googling and found that the typical threshold for both AM
and FM broadcast xmitters is 1.5:1.
https://www.google.com/#q=transmitter+high+vswr+threshold+1.5:1
Same with most HF radios that I could find. The stuff I designed for
marine use (150 watts PEP) was set to operate up to 2.0:1 because at
the time, ATU's were just appearing and the typical vessel HF antenna
was problematic (23ft vertical with a dubious ground system). Today,
1.5:1 threshold would probably work.

Note that such a threshold does NOT mean that at 1.4999:1, the radio
would work normally, and at 1.5001:1 would shut down. For (marine)
radios that are expected to work with random antennas in emergencies,
shutting down at 1.5:1 is absurd. What is normally done is to slowly
reduce the drive starting at 1.5:1 until it gets to some point below
where either the final current is too high to maintain safe
dissipation, or the voltage across the final xsistor or FET is too
high to prevent breakdown. My guess(tm) is that's about 5:1 or more
with todays radios but I'll admit that I'm guessing and haven't
actually tried it. In the past, I used a test load, that someone else
built, that would provide a resistive 2:1 VSWR at 25/37.5/50/75/100
ohms, and an adjustable phase angle with a big variable capacitor and
roller inductor. I had to be VERY careful not to accidentally tune
the inductor and capacitor to resonance, or I would end up with a
short or open load. The dials had an accompanying chart that followed
varioius constant VSWR circles around the Smith chart.

The tricky part was not making the power amp work over a 4:1 impedance
range. The tricky part was making a VSWR sensor that would be fairly
flat over the entire 2 to 30 MHz range. Another headache was when a
mismatch caused the PA to draw more current. It wasn't final heating
that caused instabilities and odd behavior. It was my worthless bench
power supply that would detect the overcurrent and protect itself by
dropping the output voltage.

So, which radio shuts down at less than 1.5:1 VSWR? I know of a few
possible candidates, but I would like to see what you've observed
first.

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

On Tue, 30 Jun 2015 17:21:08 -0700, "Wayne"
wrote:

# Since only the power that actually reaches the antenna can be radiated I
have a hard time seeing the point of matching the transmitter to the feed
line. Matching at the feed line connection point will prevent damage to the
transmitter but if # that were the main objective a dummy load would
accomplish that.

Yep. Note that the electric utility companies do not bother to match
the transmitter (generators) with the impedance of the transmission
lines and the load. That was one of the reasons that Edison and
Westinghouse has so much trouble with the experts when they proposed
electric power transmission. The experts assumed that the source had
to be matched to the load, which would cause the generators to
dissipate as much power as is dissipated in the load. Incidentally,
one reason Tesla/Westinghouse eventually went with 60 Hz instead of
133Hz, 400 Hz, or higher frequencies (which use less iron in the
xformers) was the danger of creating standing waves on the
transmission lines because of the mismatch. When the wavelength of 60
Hz (3100 miles or 5000 km) is longer than the width of the country,
it's a safe bet that there aren't going to be any standing waves.

So, why don't we run transmitters with lower than 50 ohm output
impedances? Well...
1. The gain of the PA stage would be reduced possibly requiring an
additional gain stage.
2. The current in the PA stage would increase, possibly causing the
power supply to complain.
3. The low pass harmonic filter will require physically larger parts.
4. The coax cable between the PA stage and the RF connector will need
to have a very thin dielectric to work at low impedances. Same with
the RF output connector.

Instead of dealing with these aforementioned hassles, it's probably
better to run the transmitter at some impedance that provides a
benefit and let everything else conform to that standard. That's
where the maximum power at 50 ohms for transmitters, and lowest loss
for 75 ohms (air dielectric) for CATV were derived. The rest of the
connected devices (PA, filter, antenna) simply conformed to these
standards.


--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

On 7/2/2015 6:31 AM, John S wrote:
On 7/1/2015 4:23 PM, Jerry Stuckle wrote:
On 7/1/2015 12:26 PM, Jeff Liebermann wrote:
On Tue, 30 Jun 2015 15:13:55 -0400, Jerry Stuckle
wrote:

Yes, it's most effective to match the feedline to the antenna at the
antenna connection. But it's also important to match the
transmitter to
the feedline.

This latter piece is often ignored because people will use a feedline
who's characteristic impedance matches the transmitter already (i.e. 50
ohm line for a 50 ohm transmitter).

However, there are exceptions. For instance, if you're feeding a 75
ohm
antenna (i.e. a dipole) with 75 ohm coax, a 1:1 balun at the antenna
will provide a good match (ideally, 1:1). But there will be a 1.5:1
mismatch to a 50 ohm transmitter. In this case it would be better to
have the matching network at the transmitter.

We may have had this discussion before. Matching a 75 ohm load to a
50 ohm source might be academically interesting, but the actual loss
is almost negligible. for a VSWR of 1.5, the return loss is 14dB and
the load mismatch attenuation is 0.177dB. That's about what I would
expect to lose in two coax connector pairs.

You could also feed the antenna with 50 ohm feedline and place the
matching network at the antenna. The effect would still be a 1:1 SWR,
but the lower impedance of the coax would create higher i^2R losses;
not
important if you're talking a short line, but a longer one would lower
output at the antenna.

True, but for roughly equivalent sizes of coax cables, the 75 ohm
cable has less loss and the equivalent 50 ohm cable. If you want to
handle high power, use 50 ohms. If you want low loss, use 75 ohms:
http://www.belden.com/blog/broadcastav/50-Ohms-The-Forgotten-Impedance.cfm

Note that these are for air dielectric cables.

Things are not so neat if we consider the dielectric. See the bottom
paragraph and graphs:
http://www.microwaves101.com/encyclopedias/why-fifty-ohms
Dielectric Dielectric const Minimum loss impedance
solid PTFE 2.2 50 ohms
foam PTFE 1.43 60
air 1.0 75
RG-6/u CATV 75 ohm foam coax still has slightly less loss than the
equivalent 50 ohm cable, but not as much as I've previously claimed.

This is cute:
http://cablesondemandblog.com/wordpress1/2014/03/06/whats-the-difference-between-50-ohm-and-75-ohm-coaxial-cable/

"A good rule of thumb is that if the device being connected
via coaxial cable is a receiver of some kind, 75 Ohm Coax is ideal."



Jeff, do you always miss the forest for the trees? That was an EXAMPLE.
The same would be true if you were feeding a 300 ohm yagi with 300 ohm
twinlead and a transmitter with a 10 ohm output impedance.

Transmitter output impedance does not determine SWR.


Transmitter output impedance vs. feedline impedance does determine SWR
at one end of the system. If you have a mismatch, you will have a
non-1:1 SWR.

And BTW - when calculating, you forgot about the transmitters which cut
back power to protect the finals. Many will do so even with a 1.5:1 SWR.



--
==================
Remove the "x" from my email address
Jerry, AI0K

==================

On 7/2/2015 8:43 AM, Jeff Liebermann wrote:
On Wed, 01 Jul 2015 17:23:54 -0400, Jerry Stuckle
wrote:

Jeff, do you always miss the forest for the trees? That was an EXAMPLE.

Perhaps you missed my point. I don't care about VSWR as long as the
system is reasonably efficient, does not protect itself, and produces
adequate TX power. In my world, that means I'm concerned about
losses, not VSWR. The losses involved in using 75 ohm feedline and
coax in a 50 ohm system are negligible.


I didn't miss your point. But you can't see the forest for the trees.

The same would be true if you were feeding a 300 ohm yagi with 300 ohm
twinlead and a transmitter with a 10 ohm output impedance.

One of my favorite methods of argumentation is to provide a ridiculous
and extreme example, and then use it as the basis for discussion. I
guess it's a form of "straw man" argument, where I'm now expected to
defend my point of view against your ridiculous and extreme example.
Please forgive me for not following your lead and continuing to be
more concerned with the 50/75 ohm problem:
https://en.wikipedia.org/wiki/Straw_man


I prefer to use realistic examples to show a point. But you have to
nitpick with off-topic comments.

And BTW - when calculating, you forgot about the transmitters which cut
back power to protect the finals. Many will do so even with a 1.5:1 SWR.

Could you provide me with one example of such a transmitter? I've
seen such radios on the bench, but they're usually mistuned or
misadjusted.


Many of the solid state finals amateur transmitters will start cutting
back well before 2:1 SWR. Even my early 80's era IC-720A would start
dropping power before then.

I did some Googling and found that the typical threshold for both AM
and FM broadcast xmitters is 1.5:1.
https://www.google.com/#q=transmitter+high+vswr+threshold+1.5:1
Same with most HF radios that I could find. The stuff I designed for
marine use (150 watts PEP) was set to operate up to 2.0:1 because at
the time, ATU's were just appearing and the typical vessel HF antenna
was problematic (23ft vertical with a dubious ground system). Today,
1.5:1 threshold would probably work.


We're not talking AM and FM broadcast transmitters (which are immaterial
because the antenna system is tuned to get as close to a 1:1 match as
possible - much easier with one frequency). Pretty much the same with
marine use - a very limited band of frequencies. Additionally, the
limited power on marine radios allow you to use higher power finals so
they can dissipate the additional heat caused by a mismatch.

Note that such a threshold does NOT mean that at 1.4999:1, the radio
would work normally, and at 1.5001:1 would shut down. For (marine)
radios that are expected to work with random antennas in emergencies,
shutting down at 1.5:1 is absurd. What is normally done is to slowly
reduce the drive starting at 1.5:1 until it gets to some point below
where either the final current is too high to maintain safe
dissipation, or the voltage across the final xsistor or FET is too
high to prevent breakdown. My guess(tm) is that's about 5:1 or more
with todays radios but I'll admit that I'm guessing and haven't
actually tried it. In the past, I used a test load, that someone else
built, that would provide a resistive 2:1 VSWR at 25/37.5/50/75/100
ohms, and an adjustable phase angle with a big variable capacitor and
roller inductor. I had to be VERY careful not to accidentally tune
the inductor and capacitor to resonance, or I would end up with a
short or open load. The dials had an accompanying chart that followed
varioius constant VSWR circles around the Smith chart.


I never said shut down. I said cut back. But you can't read very well,
either, can you?

The tricky part was not making the power amp work over a 4:1 impedance
range. The tricky part was making a VSWR sensor that would be fairly
flat over the entire 2 to 30 MHz range. Another headache was when a
mismatch caused the PA to draw more current. It wasn't final heating
that caused instabilities and odd behavior. It was my worthless bench
power supply that would detect the overcurrent and protect itself by
dropping the output voltage.

So, which radio shuts down at less than 1.5:1 VSWR? I know of a few
possible candidates, but I would like to see what you've observed
first.


Once again, you can't read. But I know you're just foaming at the mouth
to contradict me, as you always do. But that's OK. I know what you
are, and I'm not going to bite.

Now go away, troll, and play with your CB radios. They're your speed.
Leave this discussion to adults - who can read and understand the points
being made.

And BTW - you can also go running to your mommy and tell her the mean
old man called you a troll.

--
==================
Remove the "x" from my email address
Jerry, AI0K

==================

On Thu, 02 Jul 2015 06:12:33 -0700, Jeff Liebermann
wrote:
(...)
where the maximum power at 50 ohms for transmitters, and lowest loss
for 75 ohms (air dielectric) for CATV were derived. The rest of the
connected devices (PA, filter, antenna) simply conformed to these
standards.

Continuing from where I accidentally hit the "send" button...

It might be interesting to measure the output impedance of your HF
xmitter. All you need is a dummy load, and an RF voltmeter, RF probe
and voltmeter, or oscilloscope.
1. Turn down the xmitter RF output to some level where you won't blow
up your test equipment and so that it doesn't go into high VSWR
protect mode. My guess is about 10 watts is about right.
2. Measure the RF voltage across the output connector both with a
load (Vload) and without a load (Vno_load).
3. If measuring peak voltage, convert RMS by multiplying by 0.707. If
measuring peak-to-peak, divide by 2 and then multiply by 0.707.

Output_Impedance = 50 ohms (Vno_load - Vload) / Vload

It's been many years since I've done this, so I can't recall the range
of values that I obtained. I do recall that it was surprisingly large
and precipitated a few heated discussions in the lab. Also, the
output impedance will change with output power level but I don't
recall how much.


--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

On 7/1/2015 10:56 AM, Ian Jackson wrote:
In message , John S
writes
On 6/29/2015 3:47 PM, Wayne wrote:


"John S" wrote in message ...

On 6/29/2015 10:48 AM, Wayne wrote:
As a lead in, I use a 16 ft vertical on 20-10 meters, mounted on a
flat
metal roof. The antenna is fed with about 25 feet of RG-8, and
there is
a tuner at the transmit end.

You use a 16ft vertical as a lead-in? For what and how is that done?

Grammatically, the description of the vertical is a lead in for the
question, not an actual antenna lead.


What are the dimensions of the metal roof?

Somewhat irrelevant to my question. But it's about 20 by 35 feet.
I'm not looking for an analysis of the existing antenna.


While I'm pretty happy with the antenna, I'd like to simplify the
matching.

To what matching do you refer? You don't want to use the tuner, or is
there some other stuff you have not mentioned?

I want the tuner matching to be less awkward on some bands.
I'm willing to live with the existing high SWRs on the upper bands.


Thus, the question: what is the purpose of a 1:4 unun on a 43 foot
vertical? ( I assume the "4" side is on the antenna side.)

You wrote that you were interested in a 16ft vertical. Now it is a
43ft vertical?

Please disregard all about the 16 ft vertical. I'm asking about a 43 ft
vertical 1:4 unun.


I'd expect a better coax to antenna match when the antenna feedpoint is
a high Z (example, at 30 meters), but I'd also expect a worse coax to
antenna match when the feedpoint is a low Z (example, at 10 meters).

Is that the way it works, or is there other magic involved?

All this depends on your answers to the above questions.

So, lets begin again, with no distractions.

What is the purpose (or benefit) of using a 1:4 unun on a 43 ft
vertical.

Ok. Well, 43ft is a half wavelength at about 12MHz. The vertical will
be very high impedance at that frequency and a 1:4 unun will
theoretically bring that impedance down closer to the feed line
impedance.

Does this help?

It was been pointed out to me that the figures for feeder loss with an
imperfect SWR are only correct when the length is fairly long (at least
an electrical wavelength?). How much loss does 25' of RG-8 really have
at 12MHz, when there's a halfwave hanging on the far end?

A *resonant* half wave at 12MHz is about 36.7 feet long and it presents
an impedance of about 1063 + j0 ohms to the RG-8 at the antenna end. The
current at the antenna end is 0.0245A while one watt is applied at the
source end. This means that the power applied to the antenna is about
0.687W. So, about 68% of the applied power reaches the antenna.

So, about 32% of the power is lost in the RG-8 for this example.

Does this help?

On 7/2/2015 8:14 AM, Jerry Stuckle wrote:
On 7/2/2015 6:31 AM, John S wrote:
On 7/1/2015 4:23 PM, Jerry Stuckle wrote:
On 7/1/2015 12:26 PM, Jeff Liebermann wrote:
On Tue, 30 Jun 2015 15:13:55 -0400, Jerry Stuckle
wrote:

Yes, it's most effective to match the feedline to the antenna at the
antenna connection. But it's also important to match the
transmitter to
the feedline.

This latter piece is often ignored because people will use a feedline
who's characteristic impedance matches the transmitter already (i.e. 50
ohm line for a 50 ohm transmitter).

However, there are exceptions. For instance, if you're feeding a 75
ohm
antenna (i.e. a dipole) with 75 ohm coax, a 1:1 balun at the antenna
will provide a good match (ideally, 1:1). But there will be a 1.5:1
mismatch to a 50 ohm transmitter. In this case it would be better to
have the matching network at the transmitter.

We may have had this discussion before. Matching a 75 ohm load to a
50 ohm source might be academically interesting, but the actual loss
is almost negligible. for a VSWR of 1.5, the return loss is 14dB and
the load mismatch attenuation is 0.177dB. That's about what I would
expect to lose in two coax connector pairs.

You could also feed the antenna with 50 ohm feedline and place the
matching network at the antenna. The effect would still be a 1:1 SWR,
but the lower impedance of the coax would create higher i^2R losses;
not
important if you're talking a short line, but a longer one would lower
output at the antenna.

True, but for roughly equivalent sizes of coax cables, the 75 ohm
cable has less loss and the equivalent 50 ohm cable. If you want to
handle high power, use 50 ohms. If you want low loss, use 75 ohms:
http://www.belden.com/blog/broadcastav/50-Ohms-The-Forgotten-Impedance.cfm

Note that these are for air dielectric cables.

Things are not so neat if we consider the dielectric. See the bottom
paragraph and graphs:
http://www.microwaves101.com/encyclopedias/why-fifty-ohms
Dielectric Dielectric const Minimum loss impedance
solid PTFE 2.2 50 ohms
foam PTFE 1.43 60
air 1.0 75
RG-6/u CATV 75 ohm foam coax still has slightly less loss than the
equivalent 50 ohm cable, but not as much as I've previously claimed.

This is cute:
http://cablesondemandblog.com/wordpress1/2014/03/06/whats-the-difference-between-50-ohm-and-75-ohm-coaxial-cable/

"A good rule of thumb is that if the device being connected
via coaxial cable is a receiver of some kind, 75 Ohm Coax is ideal."



Jeff, do you always miss the forest for the trees? That was an EXAMPLE.
The same would be true if you were feeding a 300 ohm yagi with 300 ohm
twinlead and a transmitter with a 10 ohm output impedance.

Transmitter output impedance does not determine SWR.


Transmitter output impedance vs. feedline impedance does determine SWR
at one end of the system. If you have a mismatch, you will have a
non-1:1 SWR.

And BTW - when calculating, you forgot about the transmitters which cut
back power to protect the finals. Many will do so even with a 1.5:1 SWR.




Not true. Post some links to support your position, please.