CRAIG CLARK wrote:
> Bill Orr, W6SAI talked about the W1BB antenna in one of his antenna columns
> and in his Antenna Handoook on page 138. Orr was not claiming any magic
> increase in performance nor did W1BB. By having a longer than needed
> antenna, you used a capacitor to shorten it electrically, and Orr states "
> /*the ground losses of a Marconi antenna can be decreased by raising the
> radiation resistance of the antenna."*/
>
>
> This idea eventualy will go the way of an urban topband myth that turns out
> to be logical but incorrect. In the real antenna range test and subsequent
> paper presented at the 1996 NAB Engineering Conference Ron Rackley, P.E of du
> Treil, Lundin $ Rackley, Bobby Cox PhDEE, Jamers Moser, MSEE asnd Tom King,
> MSEE of Kintronics set up an FCC authorized test facility at Bluff City, TN
> with 1680 kHz transmitter and 400 watts into two types of towers 100 foot and
> 160 foot tall. Tests envolved 120 wire #10 150 foot radials and no radials
> with a single ground rod. In each case a fold unipole or six wire cage feed
> was used as a comparison to a standard series fed. The report summerizes
> that:
"No major differences in field strength between folded unipole and
series fed case were found for any of the configurations tested."
and later in the paper......
"It is commonly believed that folded unipoles are not subject to ground
losses as are series fed towers, particularly with electrically short
towers, because the wire cage dimensions and stub point can be adjusted
to yield much higher input resistance values. It is reasoned from
circuit theory that the lower input currents necessary to drive an
antenna which has a higher input resistance means that the ground
currents are reduced correspondingly and, therefor, the ohmic loses are
reduced. The flaw in this reasoning is that circuit theory only deals
with conduction currents and displacement currents are neglected. A
given level of displacement current must be present in the region
surrounding an antenna if a given height for a given amount of power,
according to Maxwell's equations. The field strength test measurements
do not challenge Maxwell's Equations, as no major difference in
radiation was found for the worse case tested, the 100 foot tower with
no ground radials."
The complete test results and commentary was online at
www.dlr.com/dlrweb/papers/nabpaper/nabpaper.htm in 2003 but recently has
not been available for download. Perhaps a request to dlr.com can get
some results if you are interested in reading the whole mater. One of
the most evident tests was to convert the cage fed antenna to a series
fed with the cage remaining but tower ground strap removes and the
bottom of the cage connected to the bottom and top of the tower
resulted in increased bandwidth compared to both the series fed vertical
and the unipole cage.The group also followed up with NEC-4.1 models
which showed that "series and folded monopole antennas do not differ
significantly in radiation efficiency for a given tower height and
ground system........"
Regards,
Herb Schoenbohm, KV4FZ
>
>
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