Topband: W1BB antenna

[Herb Schoenbohm]

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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