At 01:32 PM 9/3/2005, Tom Rauch wrote:
> > Cebik models that these get 6dB hit over a dipole. If
>they're that lossy,
> > it's no wonder they get a good match: heck, any antenna
>with a 6dB pad in
> > the feed line will get no worse than 1.66:1 VSWR.
>(nothing says that "any
> > antenna" will actually radiate, though).
>
>
>Actually the loss varies quite a bit by band. On bands where
>feedpoint Z would be low, termination resistance is shunting
>a low impedance and loss is modest. On other bands loss can
>be very high, over 10dB.
>
>What you have to do is model the antenna in EZnec 4 and look
>at average gain. You could also look at dissipation in the
>termination. Either gives an approximate idea of efficiency.
One would also look at the dissipation in the antenna wires themselves
(especially in the stainless steel variety, that might be significant..)
>Look at peak gain in the lobes and comparing kit to a dipole
>isn't a valid data point. The reason is the antenna forms
>multiple nulls on higher bands a dipole does not have, and
>so it squeezes radiation into smaller lobes than a reference
>dipole or isotropic radiator would have. The absence of
>energy in null areas that a dipole would fill means the
>antenna is less useful for overall communications, unless
>you could rotate the antenna or were working fixed
>directions and somehow managed to align narrow lobes with
>the desired directions.
L.B.'s analysis shows the patterns are basically identical, and everywhere
6dB lower for the B&W (i.e. I think he modeled a dipole of the same length
and assumed an ideal matching network).
However, you are right as to the practicalities for communications.
>You have to either use average gain or resistor dissipation
>to get a good efficiency comparison.
Which is exactly what I'm in the process of doing. Reading the B&W patent
(actually Elmer Bush's #4423423 from 1981) provides a lot of useful details.
>That particular antenna "design" actually came from someone
>in Hudson Ohio. He got it out of some old antenna article.
>It was sold to B&W by that person after he tried to market
>them for a while. Resistor dissipation was measured, and the
>dissipation figures tracked pretty well with models I made
>in Eznec. (I still have one of the original termination
>resistors used in the measurements). On some bands loss is
>more than ten dB. On some bands less than 3 dB.
>A second variation of that antenna was produced by an
>amateur antenna company called Sommer. It is a ~25 foot
>folded vertical that is terminated. It's another efficiency
>dog. It's less than 5% efficient on 80 meters.
At least with a 6 dB pad, you're only down to 25% efficient.
>A third variation is the Max Con or Max Com automatic tuner
>(not sure which name spelling).
MaxCom from Pompano Beach, FL. I had one of those in the lab for testing
back in the 80s. A nice white painted aluminum box about 3"x6"x1" filled
with some sort of opaque encapsulant. Quite lossy (heh!)... we couldn't
get funding to xray or dissolve out the potting.
> It employs a resistance to
>shunt whatever load is across the "tuner". It's claimed to
>have instant "automatic tuning", but in fact it's just the
>same old game of shunting off a large portion of applied
>power as heat in order to bring SWR into reasonable areas.
It wasn't just a resistive pad (the AC and DC impedances were different)..
perhaps there was some sort of lossy choke in there or something.
We had hoped it was some clever scheme that rectified the transmitted power
to run a conventional autotuner, but given the low selling price (a few
hundred dollars), that was unrealistic.
Actually, though, in some system contexts, the 6dB pad approach isn't all
bad. As L.B. points out, you're typically not noise figure constrained on
HF receive. It's not claimed to be a directive antenna, so that aspect of
"gain" doesn't figure into it. In some (commercial/military) applications,
turning up the amp by 6dB is a perfectly reasonable alternative. One less
box (an antenna tuner) to worry about.
Jim, W6RMK
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