I use this approach (back to back 1/12 wave transformers, which K1XX
named an "asynchronous transformer" in his article about it in the late,
great HAM RADIO magazine) with all of my HF monoband yagis at K8CC, and
see no bandwidth issues at all on 20/15/10. I've even used it on 40M
and the resulting SWR curve mimics the SWR at the beam's feedpoint.
73, Dave/K8CC
Chet, N8RA wrote:
> 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@contesting.com
> [mailto:vhfcontesting-bounces@contesting.com] On Behalf Of David Pruett
> Sent: Friday, May 08, 2009 12:43 AM
> To: vhfcontesting@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@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
>
> _______________________________________________
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> VHFcontesting@contesting.com
> http://lists.contesting.com/mailman/listinfo/vhfcontesting
>
>
>
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