[TenTec] Antenna Matching Weirdness
John Crux
patents@dx0man.prestel.co.uk
Thu, 19 Oct 2000 22:16:01 +0100
Your question may be of much greater general relevance than you think.
Its a situation which is very easy to get into without really trying
too hard.
Firstly, you have not done anything dumber than many folk do all the
time. The main reason I have an ATU is to fool my rig into thinking I
have a separate, perfectly matched antenna for each band..
But I'm interested in your exact setup. I guess that one jumper lead
goes from your rig to the ATU. The length of that lead should not be
critical in any way (unless you have a plug or cable fault - you did
check for shorts/intermittent connections to those lousy 259s ???) The
ATU is there to ensure that your rig sees something reasonably close to
50 ohms, with minimal reactance. So when you have a near 1:1 match,
the exact length of your 50 ohm cable jumper lead from rig to ATU ought
not to make much, if any difference. If you are now unable to get a
1:1 match on one band, then its time to look beyond the ATU. What
exactly does the second cable do for a living ?? I guess it connects
directly to the MFJ antenna.
The actual impedance/reactance values present at the base of the MFJ at
any specific frequency are highly unlikely to be exactly anything in
particular .... it depends on too many variables, like height above
ground, proximity of buildings/metalwork, etc etc. In other words, the
antenna impedance on any band is anyones' guess. MFJ obviously had
some target values in mind, but they are not carved in stone in a real
life ham situation. (I assume you have no new fault on the MFJ itself.)
Anyway, its not designed for the WARC bands.
If you now connect 50 ohm coax to that set of unknown impedances, I
can practically guarantee that at the shack end of that coax, you will
not see 50 ohms. What you will see is a set of impedance values that
are also anyones' guess. (OK, unless the coax is exactly an electrical
half wave long at the frequency of interest - when the impedance will
be the same as at the antenna base). The job of the ATU is to make the
oddball set of impedances arriving in the shack look like 50 ohms
resistive, at the INPUT to the ATU. The ATU has zero effect on the
impedance(s) of the antenna itself. Its job is to make them as
acceptable as possible. .
Unfortunately, it seems that now you have changed the cable length to
the antenna and as you also changed the operating characteristics of
the cable itself, so you also changed how it behaves when it sees a
particular mismatch.. In this case you lost a match on 12m. The good
news is that your better quality cable has probably reduced your cable
losses due to SWR.
But on 12m it looks like you now have a condition that the present ATU
cannot handle. A better (maybe more rugged) ATU might work. Before
jumping down that expensive escape route, make up another patch lead so
that you can try extending the antenna cable length by at least a
couple or three feet. Then check if you can see any improvement on 12m
and what effect it has has on other bands. It should be possible to
find a compromise length that can be used on all bands, but on some
bands you will surely have to tolerate a narrower bandwidth than on
others - you may need to retune more often as you move across the band.
This is OK; you are after all using the MFJ on a band it was never
designed for - and why not ??
Its generally perfectly acceptable to operate coax with a high SWR as
long as you keep the length to a minimum, and as long as your ATU gives
you an acceptable match to the rig - an SWR of up to 1.5 : 1 is usually
reasonable.
I have used a coax fed, ground mounted 40m ground plane on 20m. The
mismatch at the antenna base and at the shack end of the coax was
awful, but I got my ATU to handle it so the rig thought it was still
seeing about 50 ohms. A friend in VK told me it was technically
imposssible to operate a 40m GP on 20m like that.. I simply asked him
how it was that he could copy me Q5 12000 miles away if it was
impossible. It actually worked quite well but the tuning was sharp.
I now use a 40m inverted V dipole on all bands. It is fed with 90
feet of ladderline, a 4:1 balun hung outside my home, and 25 feet of
RG-8 to the TT 253 ATU. The tuning is very sharp on 20m and on 80m. Its
much less critical on 30/40/15. Its tolerable on 10/12/17. Obviously
the SWR is NOT 1:1 on any band between the ATU and the balun. and it is
not 1:1 on the ladderline. However, it does look pretty close to 1:1
from the Herc II to the ATU, so the Omni-V and the Herc are happy. The
ladderline has low loss, even at high SWR. The short length of coax is
probably a bit more lossy, but it works and thats all that matters..
How lossy ??? Dunno. The losses cannot be all that significant. I have
worked 184 countries this year using just my 10 watt Elecraft K2 (wash
my mouth out) barefoot. I hope to make 200 by the year end.
For multi-band operation, with any one antenna, a high SWR may be your
best friend, as long as you accept that you may need to play with the
coax length and you may need to retune the ATU when you move around.
Hope that makes some sense, or do I need to climb into my flameproof
suit (again).
73 John G3JAG
On Thu, 19 Oct 2000, Alan Bryant wrote:
> I realize that by posting this, I'm probably going to look like an idiot,
> but I'm sincere in wanting to know what's going on with something.
> Hopefully someone smarter than I am can provide an intelligent answer. So,
> from the "there are no dumb questions" department...
>
> I use a multiband MFJ vertical for an antenna (it's all I have space and
> neighbor tolerance for), and a small antenna tuner with it to broaden its
> bandwidth. The antenna is designed for 40m through 10m, plus 6m and 2m,
> EXCLUDING the WARC bands.
>
> When I initially set-up my station, the only short coax patch cables I had
> with PL-259s on them were made of RG-58 coax, but since it was only a
> couple of 2' cables, I used them anyway. All seemed to work well enough,
> and the tuner also allowed me to match with low SWR on the WARC bands in
> addition to the others. While I know the antenna is far less than optimal
> on the WARC bands, I've managed to work some DX with this set-up on 12m
> with relative ease.
>
> I recently made new patch cables using RG-8 foam core coax, cut to the
> exact lengths I needed. Not surprisingly, the configuration of the three
> tuner knobs was now different to achieve low SWR. But suddenly, I can no
> longer successfully match on 12m at all. The lowest SWR I can get there is
> about 3:1, when I could get it clear down to 1:1 before.
>
> Understanding that trying to use this antenna on the WARC bands is probably
> dumb to start with, I'm still left wondering why I could match it
> successfully with crappy, inappropriate coax (RG-58), and now I can't when
> using quality coax connections of the proper type (RG-8).
>
> Any thoughts on this are welcome (except those, perhaps, telling me I
> shouldn't expect it to work to begin with).
>
> Thanks.
>
>
>
> ---
> Alan Bryant, WDØEVX
> Denver, Colorado
> http://www.moonworks.com/wd0evx
>
>
> --
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