Topband: MA160V - Performance?
mathenyr at marietta.edu
Thu May 11 10:05:26 EDT 2006
In short--THERE IS NO FREE LUNCH!---
207 Gibbons Place
Marietta Ohio 45750
On Thu, 11 May 2006, Tom Rauch wrote:
> > It says that adding radials may make the radiation
> > resistance below 50 ohms.
> > On page 10 it says that enhancing the radiation can be
> > done by adding
> > radials. Cushcraft says 16 radials 50-60 feet will help,
> > but will lower the
> > radiation resistance requiring a matching network such "a
> > shunt inductor,
> > fixed-ratio Ununn, transmission line transformer, or
> > adjustable L-network."
> > What Joe has done with his antenna is doubled the
> > "recommended enhancement".
> > With an SWR of 2.5 to 3.0 the radiation resistance has
> > dropped from 50 ohms
> > to 17-20 ohms, so about 30 ohms of ground loss resistance
> > has been
> > eliminated. This increases the efficiency by about 4 db.
> > Remember that the MA160V is only about 24 feet from base
> > to the loading coil
> > and top loading wires and vertical "stinger" which gives
> > about 0.05
> > wavelength effective radiator length.
> > In summation, I personally believe this antenna will be a
> > compromise at any
> > power level over any radial system compared to typical 160
> > meter antennas in
> > use.
> I fully agree with George.
> Assuming a vertical .05WL (18 degrees) high the maximum
> possible current loop radiation resistance for any possible
> configuration of loading is 1580 times the height in
> factions of a wavelength squared, or 3.95 ohms. Rr = 1580
> times square of He That is the very MAXIMUM radiation
> resistance possible. It will always be less than that.
> With base loading radiation resistance and uniform diameter
> element radiation resistance would be 1/4 of that value, or
> about 1 ohm.
> If you have a simple series-fed 27-foot radiator and no
> matching and have a 50 ohm feedpoint resistance and top
> loading the very maximum efficiency would be 4/54= .074 or
> 7.4%. That's the best you could ever expect to do. You have
> at least 11dB loss and probably more. There isn't any way
> possible around that.
> Manufacturers almost always focus on SWR. They do that
> because people can easily notice SWR, but most will not
> notice 10dB of loss.
> SWR sells antennas, but high efficiency makes a good signal.
> 73 Tom
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> Topband at contesting.com
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