To clarify...
1. Increasing the radiation resistance DOES lower the ground loss, since
that loss resistance is a smaller portion of the total feedpoint resistance.
2. A folded unipole transforms the impedance at the feepdoint, but it DOES
NOT increase the radiation resistance. The study referenced by Herb KV4FZ
was done to finally put to rest two very different, and erroneous, views.
First, there were those who confused higher feedpoint resistance with higher
radiation resistance (and higher efficiency). Also, there were some in the
broadcast industry who believed that shunt- or skirt-feeding a grounded
tower was less efficient than a series-fed installation.
3. In an inverted-L, the relatively short vertical section limits the
radiation resistance improvement that can be made by lengthening the top
section. I suspect that W1BB's claim of an optimum length was based on
experiments that combined this small performance improvement with easier
matching to a higher feedpoint impedance. Even if it is entirely based on
ease of matching, he certainly found that additional length was detrimental
to performance.
4. There are numerous antennas in the amateur literature that have schemes
to raise the impedance at the feedpoint, but do not change radiation
resistance. Especially in pre-NEC days, well-known authors would
occasionally fall prey to the hype, including W1FB, W6SAI and W1ICP. Because
these writers were authority figures, some of those myths and
misunderstandings continue today...
73, Gary
K9AY
> 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:
[...etc.]
> Herb Schoenbohm, KV4FZ
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