[TenTec] "Tuning" antennas; baluns

[Bill Coleman AA4LR]

On 9/23/97 4:58 PM, Chester Alderman at chestert@pressroom.com wrote:

>At 11:44 AM 9/23/97 -0400, you wrote:
>>On 9/23/97 11:04 AM, rohre at rohre@arlut.utexas.edu wrote:
>>
>>>In fact, I hope all will agree or will measure,  that even with an antenna
>>>matching device, (which for years was mistakenly called a "tuner"), you 
still
>>>have standing waves on the transmission line, and the matching device is 
just
>>>there to TRANSFORM a complex impedance at the shack end of the line to a 
>>>value that more closely matches your transmitter.
>>
>>Not quite. In my book, a transmatch produces a "matched" or resonant 
>>condition on the antenna "system" -- which includes the transmission 
>>line. The resulting conjugate match is just as resonant as any other type 
>>of antenna system.
>
>Which book is it you published? I resist the urge to ask you to explain 
>the meaning of a 'conjugate match'. If what your saying is correct, why 
>isn't it called a 'antennamatch' instead of a trans(mission)match? Just 
>wondering!

Got an ARRL Handbook? (No ham should be without one)

Mine is the 1991 ARRL Handbook, page 16-11. The section titled "The 
Conjugate Match and the Z(0) Match"

I'll quote a bit: "If a generator, a load and an ideal lossless line 
connecting them all have the same Z(0), both a Z(0) and a conjugate match 
exist. (A Z(0) match is a special condition of the conjugate match).

"If the Z(0) load is now replaced by another, such as antenna having an 
impedance Z(L) = R + jX, we have two mismatches at the load, a Z(0) 
mismatch and a conjugate mismatch. The Z(0) mismatch creates a reflection 
having a magnitude rho = ( Z(L) - Z(0) ) / ( Z(L) + Z(0) ), causing a 
reflection loss rho^2 that is referred back along the line to the 
generator. This in turn causes the generator to see the same magnitude of 
Z(0) mismatch at the line input. This referred mismatch causes the 
generator to deliver less than its maximum available power by the amount 
equal to the reflection loss. There is no power lost in the line -- (it 
was lossless, remember?) -- only a reduction in power delivered. All of 
the reduced power delivered is absorbed by the load.

(Stay with me)

"If a matching device is now inserted between the line and the load (ie 
at the antenna), the device provides a conjugate match at the load by 
supplying a reflection gain which cancels the reflection loss, so the 
line sees a Z(0) match at the input of the matching device. Since the 
line is terminated with a Z(0) match, the generator now sees a Z(0) match 
at the line input, and will again deliver its maximum available power 
into the line, to be completely absorbed by the load.

(Now, pay CLOSE attention to this next paragraph)

"However, if a Transmatch is inserted between the generator and the line 
instead of a matching device at the load, the Transmatch provides a 
conjugate match at the line input, and the generator sees a Z(0) matchat 
the Transmatch input. At the load, the line now sees a conjugate match, 
but also sees a Z(0) mismatch. The Z(0) mismatch again causes a 
reflection loss that is referred back to the line input as before. With 
the Transmatch at the line input, the reflection gain, which again 
cancels the reflection loss, sheilds the generator from seeing the 
referred Z(0) mismatch again.

(Don't fall asleep, or you'll miss my whole point!)

"The reflection gain of the Transmatch also creates the conjugate match 
at the load, and thus enables the load to absorb the maximum available 
power delivered by the generator. Since the generator sees a Z(0) match 
at the Transmatch input, it continues delivering ITS MAXIMUM AVAILABLE 
POWER THROUGH THE TRANSMATCH, AND THE POWER IS STILL COMPLETELY ABSORBED 
BY THE LOAD BECAUSE OF THE CONJUGATE MATCH." (emphasis mine)

In other words, the use of a transmatch has NOTHING TO DO WITH antenna 
efficiency. You can deliver power just as efficiently to a non-resonant 
antenna as you can to a resonant one. It does not matter if the matching 
components are at one end or the other of the transmission line.

>>True, there are so-called "standing waves" on the transmission line. But 
>>the presence of these waves has next to nothing to do with the resonance 
>>of the antenna system. I'll give you an example -- a "resonant" dipole in 
>>free space has a feedpoint impedance of around 70 ohms. Hook 50 ohm coax 
>>to it and you have a mismatch that results in about 1.5:1 SWR.
>
>Sorry, they are not 'so-called', they actually exist and if you have the 
>correct equipment, they are measurable. But you are correct, standing 
>waves have nothing to do with the resonance of a dipole.
>
>>Standing waves are only a problem in lossy transmission lines -- such as 
>>coax. Open wire has so little loss that the standing waves can be safely 
>>ignored.
>
>I think what you really meant to say is that standing waves are only a 
>problem in lossy transmission lines....

I meant just that -- that standing waves are a problem in lossy 
transmission lines. Period.

>...as long as the phase of the wave on 
>each wire is exactly 180 degrees out of phase.

Huh? You're confused.

> And if you think standing 
>waves are 'only a problem in lossy transmission lines, please tell us what 
>happens to your beautiful TenTec rig when you run 100 watts into a 50:1 
>STANDING WAVE RATIO. Thank you.

(I wish I had a TenTec) 

'cuse me, that 50:1 SWR condition is what the Transmatch is for. It takes 
care of it. My hypothetical TenTec rig sees a perfect 50 ohm non-reactive 
load.

>>If you had a lossless 50 ohm coax, you could hook it to that 70 ohm free 
>>space dipole and safely ignore the mismatch. Of course, with lossly coax, 
>>that 1.5:1 SWR is probably much lower (feedline losses apply equally to 
>>the reflected energy) at the transmitter -- probably around 1.2-1.4:1. 
>>Low enough that you might ignore it anyway.
>
>Hmmmmm..I love to learn these things... But what actually happens is that 
>feedline losses increase with SWR increase.

That's true as well. But that wasn't what I was talking about. Consider a 
lossly transmission line. Not all of the energy supplied by the generator 
reaches the load. Consider also that not all of the returned energy from 
the load mismatch reaches the generator. Since the magnitude of the 
returned energy is proportional to that reaching the load, and the 
returned energy incurs loss, a lossy transmission line will show a much 
lower SWR at the transmitter than at the antenna.

Indeed, at high VHF and UHF, you can make very nice dummy loads by 
putting a connector on several hundred feet of RG-175 or RG-58 and 
leaving it unconnected at the other end.

>When you see the statement 
>that "RG213 has 1.5Db loss per 100 feet at 30MHz" if you read the notes or 
>the 'fine print' it will tell you that this figure is true ONLY if the 
>LOAD at the opposite end is 52 ohm +/-j0 ....

Now, I hope you are beginning to see that all this sensitivity to SWR on 
the transmission line has NOTHING to do with the resonance of the 
antenna, and everything to do with the LOSSY nature of coax. Since coax 
is so lossy, we really can't use it effectively in a conjugate matched 
system with a Transmatch. That's why the emphasis on obtaining a Z(0) 
match! It's the damn coax, not the antenna!

>>>I hope this helps divide the concepts into their proper areas.  Antennas
>>>resonate depending upon their physical structure and dimensions.
>>
>>No, antenna "systems" resonate depending on their ELECTRICAL 
>>characteristics. A shortened dipole is no less resonant than a full-size 
>>dipole, yet its dimensions are different.
>
>ABSOLUTELY CORRECT!!! BUT a shortened dipole is resonant at say 30 MHz 
>while a full size dipole is resonant at 3.5 MHz!!

By shortened, I meant that a dipole having shorter dimensions, but still 
having the same resonant frequency. The requisite electrical length being 
supplied by loading coils somewhere along its length.

>Please try measuring the 
>resonant frequency, i.e., the frequency where you have R +/-j0 impendance, 
>of a dipole that is 66 feet on each side; then measure the resonant 
>frequency of a dipole that is 6 feet on each side. And then please post 
>your results. If a 12' dipole is the same resonant frequency as a 132' 
>dipole, all my beautiful TenTec gear is up for sale and I'm going to surf 
>the internet.

I can make a 12' dipole just as resonant on 3.5 MHz as a 132' dipole, by 
inserting suitable coils. (At 12', the entire length of the dipole will 
probably be a coil -- this technique is usually referred to as a 
helically wound antenna)

>>There's nothing magical about certain antenna dimensions or "resonance." 
>>The overriding issue is concern for energy LOSS. Once you understand the 
>>nature of energy loss in an antenna system, it is a simpler matter to 
>>reduce such losses.
>>
>I really don't want to make any ememy's so I will not comment on this last 
>paragraph. Honest, I'm not going to say another word, ever though I've 
>erased four responses to it and none of them are decent enough to print.

I'm sorry I left you responseless.

--

(I suppose I'm so picky about this subject because I heard of a ham who 
lived nearby who fell off his tower trying to make his antennas perfectly 
"resonant.")

Bill Coleman, AA4LR, PP-ASEL        Mail: aa4lr@radio.org
Quote: "Not in a thousand years will man ever fly!"
            -- Wilbur Wright, 1901


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