Paul,
While the technique you describe certainly works, it has the downside
that as the line becomes longer in terms of wavelengths (so that it
contains more and more 1/2 wavelength sections), the bandwidth over
which a good match is maintained is narrowed.
One of my contesting friends W8MJ, tried this on HF with a ~350' run of
1/2" CATV hardline to a TH6DXX tribander on his back tower. The length
was trimmed with an SWR analyzer to be multiples of half wavelengths on
all the bands (20/15/10). On 10M the SWR bandwidth was noticably worse
than either what was shown in the manual, or by other TH6's at his
station (fed with 50 ohm line). Putting a 50:75 un-un at each end made
the SWR look like the other antennas.
It should be noted that the line was some 25 half-waves long at 28 MHz.
When considering VHF use of this technique, bandwidth may not be an
issue, but then the capability to cut the line accurately to the correct
length might be.
Dave/K8CC
Paul N1BUG wrote:
> Ed,
>
> Since you have already run the lines and probably cut them to some
> preferred length, you may not want to do this. But for what it's
> worth I don't bother with the matching sections any more. If you cut
> the line to a multiple of a half wavelength (which must be done
> experimentally unless you have access to some fancy test equipment)
> it will repeat the impedance of the load. 50 ohm antenna, CATV line
> cut to a multiple of a half wavelength = 50 ohms at the shack end of
> the line.
>
> 73,
> Paul N1BUG
>
>
>
> Ed Stallman wrote:
>
>> Hello Group, I ran two length of 3/4 inch 75 ohm CATV cable 200 foot each
>> to my newly installed tower. one each for 6 and 2M ! What I'm looking for
>> is an impedance transformer to match my radio/antenna, I believe a 62 ohm
>> transformer. I remember seeing these somewhere made from copper water pipe,
>> I think they were 1/4 WL? I've searched on the internet with no luck! can
>> someone point me in the right direction with a URL or publication, ARRL VHF
>> book. even the formula would be helpful for knowing the outside and inside
>> diameter of the tubing.
>>
>> Thanks Ed N5BLZ
>>
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>
>
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