[Elecraft] Re: [Icom] Opinions re: HF Verticals?

[Charles Greene]

Larry and All,

I'm not disagreeing with you Larry, and here's a story to illustrate your 
point.  I recently built and tested a portable vertical in my yard for use 
at a special event site in the field.  The design of the antenna is not at 
issue here so I won't get into it.  I carefully tuned the antenna by 
adjusting the length of the radials and radiator for a Z of 50 ohms and 
resonance by measuring R + jX at the base of the antenna for the point at 
which the X component crossed from + to -.  BTW, this is not the point of 
lowest SWR, but close to it.  I was using an Autec VA1 which measures SWR, 
R, X and a bunch of other stuff.  On this meter you can't get X to read 0, 
but it will jump from like a -8 ohms to a +8 ohms as you tune across the 
resonant frequency.  At this point the SWR read 1.02:1 at 14.2 MHz and R 
read 49 ohms.  I was using 84' of LMR-400 coax (50 ohm low loss stuff) in 
series with about 16' of RG8X and an couple of antenna switches to the 
rig.  In the shack, the SWR read 1.14:1 at 14.2, and R was 58 ohms using 
the VA1.  My SWR/Power meter read an SWR of 1.0:1 in the shack. I couldn't 
get it budge off zero reflected power.  Now that SWR and R was nothing to 
complain about and my K2 antenna tuner made short work of it, but I 
wondered why the readings were not repeated in the shack.  I attributed it 
to the length of the coax.  Then a couple days ago, I ran across an article 
by Frank Witt, AI1H in ARRL Antenna Compendium, #4 entitled "Broadband 
Matching Transmission Line Resonator."  Basically what Frank was doing was 
to "tune" the antenna system slightly by varying the length of the 
transmission line, and thus broad banding it.  In my case, I was detuning 
the antenna system slightly (where the system is the combination of the 
antenna and the transmission line) by using a random length of transmission 
line.  In order to get the same results in the shack as at the antenna you 
need an electrical 1/2 wave transmission line or multiples there-of with 
the same impedance as antenna.  Morale of this posting:  1. Don't believe 
your SWR meter, 2. Read Frank's article to see how varying the length of 
the coax "tunes" your antenna system, and 3. As long as your SWR is low but 
not exactly 1:1, don't worry about it as losses are low and the rig is happy.

Sequel to the story.  At the special event site the environment was 
different and the antenna resonance was way off.  I retuned it by varying 
the length of the radiator only as I didn't have time to vary the lengths 
of the radials too, so the SWR at resonance was about 1.2:1.  I didn't 
bother to read the Z at the station as the SWR/power meter said 1:1 and at 
this point in time I couldn't do anything about it.  Morale to the 
sequel:  Don't spend too much time tuning a portable antenna at home as you 
will probably have to redo it in the field anyway.

At 11:25 AM 10/15/2002 -0600, you wrote:
>At 02:21 AM 10/15/02 -0500, sjolin wrote:
>>  Changing the length of coax used
>>to feed an antenna will change the swr at the rig. I've seen cases where the
>>swr will change just by adding an extra couple feet of cable in the shack.
>
>That is incorrect -- if your "SWR Meter" is indeed measuring SWR. The 
>complex impedance seen by the rig (or tuner) will change depending on the 
>length of coax, but not the "true" SWR. For an insight into this, read 
>Chapters 7, 8 and 9 in "Reflections II" by M. Walter Maxwell, W2DU. A most 
>illuminating book that should be read by every "technically oriented" ham...
>
>73,
>Larry W1HUE/7
>
>PS - PLEASE lets DON'T start a big thread on SWR, complex impedances, etc.!



73, Chas, W1CG
K2 #462