[Antennas] Re: B&W window antenna

2002年4月03日 20:19:56 -0600

I could not agree more, Danny. Broadcast engineers say, basically, anything below
3:1 is acceptable (and if you look at the loss figures, that's certainly true).
Today, part of the mythology is based, simply, on the fact that solid-state finals
shut down with higher that about 1.8:1 SWR. That, I thought, was why we had
antenna tuners! Certainly not to "make antennas better," since a tuner has no
effec on the antenna at all.
A second important factor is that almost everyone today feeds antennas with coax,
and coax (as compared, say, to single wire or twin-lead feed) has BIG losses with
high SWR (by which I mean 4:1 or greater, typically, if it's good coax). If you're
willing to forgo the convenience of coax, SWR is meaningless as a measure of
anything, and even with coax it's meaningless as a measure of how your ANTENNA
performs.
In the past--as in my youth, 50 years ago--the reason why people sought low SWR was
quite simple; hams knew no other way to evaluate an antenna system, so they used
what they had, and SWR was it. My friend Lew McCoy (who was of an even earlier
generation than me) was fond of pointing out that EVERY antena used before about
the end of WW II typically had an SWR of 8:1 or worse (better?). And of course,
they were fed with #12 wire or twin lead, with losses at that SWR of almost zero.
So, (1) if your rig matches nicely to the load, perhaps because of a tuner (alomost
lossless, I might add), or (2) you're using virtually lossless feeder, or (3) your
coax run is short (and the coax vy high quality), SWR is about as irrelevant as .
. . well, some of you who are my age will remember comments about things on a bull.
73, Jan N0JR
Dan Richardson wrote:
> Why oh why do so many judge an antenna by the SWR? No matter what the
> experts say AND PROVE... Still solely judged on SWR?
>> Danny, K6MHE
>> At 07:01 PM 4/3/2002 -0500, you wrote:
> >Just some of my own input here.
> >I had the chance to help a local ham try to get his B&W window antenna to hum.
> >We tried everything and the best SWR on 20 meters was 4.1 to 1.
> >We then tried a tuner and could not get below 1.7.
> >We added grounds and 2 more radials.
> >We then tried the 40 meter and the instructions said to leave the wire for
> >the tap OFF or hook it at the end of the coil.
> >Same problem.
> >SWR was better at 3.1 but at least we got it down flat with the tuner.
> >But a tuner should not be needed.
> >Now my home brew window antennas I had great difficulty finding the sweet
> >spot on the coil for a low SWR but I did find it and the SWR was 1.3 and also
> >was taken down lower by moving the radial..
> >We went back to the B&W and added 2 turns to the coil and we moved the tap
> >just short of 1 full turn.
> >BINGO!! we had a 2.1 SWR.
> >Playing around we got as low as 1.4
> >I think I'll keep with my 9 foot job I built the first time.
> >Tomorrow B&W gets a call and I want to know why we had to add 2 turns or if
> >it was a factory defect.
> >I have not seen the MFJ but it has a 4-1 balun built into it but I can't seem
> >to get any info on it.
> >So if you feel the need for a short antenna and want to work DX build your
> >own or just use a mobile stick..
> >The coil on my home brew has 50 turns and tapped appx 2.5 turns up from the
> >bottom.
> >That gives me a flat SWR and I have a full 75 KC on 40 meters and the SWR
> >climbs up to 1.3.
> >Close but no cigar.
> >Remember if you want to build one they work great but the taps are very fussy.
> >I used a needle to tap # 18 GA RS speaker wire.
> >I has a noise bridge in line and when that started to settle down I put the
> >SRW meter in line.
> >A lot of fun and you can bag world wide DX with them as many have.
> >But B&W in the past had a coil for each band and I feel thats where they
> >screwed up.
> >I guess as i'm not an EE.
> >I just assume.
> >As it stands right now i'd rather have a coil for each band.
> >Anyone have this problem with the B&W?
> >SAVE$$$$$$$$$$$$$$ and build.
> >Bob
> >AF2Qrp <tm>
> >- - -
> >
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