[VHFcontesting] Dealing with the mismatch of using hardline coax

[Chet, N8RA]

Yes, I can see how that could give some interesting effects on HF, where the
desired band coverage is a large percentage of the nominal operating
frequency 

E.g. desiring 700 kHz out of 28 megs
But at say, 2M, where you may only want 400 kHz out of 144 megs that's not
much. 

I've not done the detailed math to compare the approaches, but the article I
referenced about the twelfth wave matching approach includes a graph showing
that it provides worthwhile improvement in the line SWR out to +/- 20% or so
from the cut frequency for a 50 to 75 ohm situation. That would say it
should cover the entire 6 or 2M band, and a VHF contester would not even
need that much. 
73
Chet, N8RA

-----Original Message-----
From: vhfcontesting-bounces at contesting.com
[mailto:vhfcontesting-bounces at contesting.com] On Behalf Of David Pruett
Sent: Friday, May 08, 2009 12:43 AM
To: vhfcontesting at contesting.com
Subject: Re: [VHFcontesting] Dealing with the mismatch of using hardline
coax

James Duffey wrote:
> Another technique is to use a half wavelength or integral multiple of 
> half wavelengths of the 75 Ohm coax. 
>
> --
> KK6MC
> James Duffey
> Cedar Crest NM
> _______________________________________________
> VHFcontesting mailing list
> VHFcontesting at contesting.com
> http://lists.contesting.com/mailman/listinfo/vhfcontesting
>   
One problem with this technique is that as the feedline becomes many 
half waves long, the bandwidth over which the match is good gets pretty 
narrow.  One of my friends used this technique with about 350' of CATV 
hardline to feed a TH6 tribander and on 10 meters (where the feedline is 
approximately 25 half waves long) the match was only good for a couple 
hundred KHz.  He put Amidon 50:75 un-uns on both ends and the match 
started to look like what the HyGain manual claimed.

And it's necessary to be pretty close to right on to make this work 
right.  Here at K8CC, I had an 80 meter four-square phased array fed 
with a random length of CATV hardline estimated to be around 270' long.  
The SWR showed 1:1 across the band when measured at the Comtek phasing 
box, and it was 1:1 at 3800 KHz but 2:1 at 3500 KHz.  Again, a pair of 
Amidon 1:1 un-uns made the SWR in the shack look like the SWR at the 
load.  But keep in mind, this was at 3.5 MHz where the line wasn't all 
that many half-wavelengths.

I would think that on VHF it wouldn't be too hard to wind up with a 
feedline many half waves long.  Even then, it might be possible to use 
this technique but you might have to carefully prune the feedline to 
optimize where the best match is centered.

I"m not saying it can't be made to work, but there are issues in play 
with this approach.

73, Dave/K8CC