[Mobile-Portable] 10 meter mobile

[W6OAL@aol.com]

Sean,

       Ah yes, experimentally or scientifically, on 10M (mobile), you need to 
run a half wave of coax between your rig and the antenna. I'm sure your 
dilemma will start a thread by all the would-be-experts. This is really a simple 
impedance matching problem once you think about it. And, you really don't need 
to use a Smith Chart or the Smith Chart Eliminator Program to figure it out.
       Yes, I was subject to this very thing a few years ago when an old time 
ham put the bug in my ear even though I had been a ham for 40 years at the 
time and was a graduate RF Engineer. Take into account the Vp of whatever coax 
you use and cut it to a half wavelength. Your problem will be gone (should be 
gone), unless you have a grounding or bonding problem (damn plastic cars of 
today). This situation will be most prevalent on ten meters for whatever reason. 
I have to wonder if CBers ever had VSWR's lower than 3:1 because surely this 
situation would have presented itself to them also. Your CB antenna has a choke 
to ground from the feed point I would imagine, making the normal 36 ohm 
vertical a 50 ohm antenna system. Your rig has a fixed output impedance of 50 ohms. 
Across a random length of coax the 50 ohm impedance of the rig is not 50 ohms 
at the antenna. Remember the source impedance down a coaxial line repeats 
every half wave length. I would say that a 3:1 indicates that the combination of 
the 50 ohm antenna and your present length of coax is presenting, to your rig 
or your VSWR meter, a load impedance of somewhere around either 17 or 150 
ohms.
       This situation will present itself (sometimes) between an exciter and 
a linear amp and the coax between the two has to be trimmed. I saw this happen 
at Bahrain when stationed over there in the '60's. Ian Cable - MP4BBW has 
just received his brand new Collins S-line. He'd invited a couple of us hams from 
the ship up to his place to help him set it up. All was fine except on 15 
meters. Between his transmitter and the 30S-1 the VSWR at the transmitter was 
horrible no matter how we tried to tune it. Not being an engineer then, I picked 
up the 'installation manual' and found a place in it that indicated - between 
transmitter and amplifier the coax needed to be 15.5 feet long so that the 
output impedance of the transmitter would be present at the linear. When we did 
this all was fine.
       For what it's worth, and please let me know if the half wave of coax 
between your rig and antenna makes the difference between 3:1 and a 1.0:1. BTW, 
if you can't make the run with a half wavelength of coax than make it a full 
wavelength or even multiples of a quarter wavelength at 10 meters.

GL CU 73, Dave...

Olde Antenna Lab of Denver
Dr. David A. Clingerman, Sc.D. (CEO)

"There is no such thing as 'reality', there is only perception!"


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