[TenTec] "Tuning" antennas; baluns

[Chester Alderman]

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!

>

>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, as Stuart so aptly explains, the physical dimensions and it's proximity to ground ARE THE ONLY THINGS THAT DETERMINE 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....as long as the phase of the wave on each wire is exactly 180 degrees out of phase. 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.

>

>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. 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 ....

>

>>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!! 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.

>

>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.




Chester Alderman

<color><param>0000,0000,ffff</param>W4BQF -- Tom

</color><color><param>ffff,0000,0000</param>chestert@pressroom.com

</color>


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