"Richard Harrison" wrote in message
...
John wrote:
"I`ll go back and try again."
John has the best help there is in Roy Lewallen, the creator of EZNEC.
I agree wholeheartedly.
The idea of breaking the behavior of a folded dipole or unipole into its
differential (transmission line)-mode and common (antenna)-mode
behaviors goes back according to Paul H. Lee in "The Amateur Radio
Vertical Antenna Handbook" to W.V. Roberts, "Input Impedance of a Folded
Dipole", RCA Review, Vol.8, No.2, June 1947, p. 289.
Around the 1/4-wave length, the folded monopole`s resistance is steadily
rising with frequency. High radiation resistance as compared with loss
is good. This happens with the open-circuit 1/4-wave vertical too.
This is what I'm trying to see using EZNEC. I agree with the resistance
trend, but I keep seeing capacitive reactance below 1/4-wave resonance and
inductive reactance above 1/4-wave resonance.
Around the 1/4-wave length, the folded monopole undergoes an abrupt
change from inductive reactance when it is too short for resonance to
capacitive reactance when it is too long for resonance. The open-circuit
whip undergoes a similar change but it has a capacitive reactance when
it is too short for resonance and an inductive reactance when it is too
long for resonance..
I see no difference in the trends.
One contributor to this folded monopole thread said he found a coil
shunted across the feedpoint of an Andrew Corporation folded monopole.
On page 26-12 of my 19th edition of the "ARRL Antenna Book" is described
a matching technique using such a coil. It`s called the "helical
hairpin" (with tongue in cheek). This method seems convenient, in
conjunction with length adjustment of the folded monopole, to get a 50 +
j0 impedance at the specified operating frequency. I am not privy to
Andrew`s actual practice as we just placed the orders and the antennas
worked as advertised.
Figure 17 on page 6-9 of my 19th edition of the "ARRL Antenna Book" is
very similar in appearance to the Andrew Corporation folded monopole.
There is a lot of good information in the Antenna Book on folded
antennas, and more.
Best regards, Richard Harrison, KB5WZI
My copy of the book is the 18th edition.
John
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