[HCARC] Answers and More Questions IRT Kerry's Comments

[Kerry Sandstrom]

Gary,

I'll try to cover the different topics separately.

You have described it correctly.  It is 2 dipoles for 2 different 
frequencies at 90 degrees to each other connected to the top of a 15 foot 
approximate length piece of coaxial hard line.  I can't tell if it is 50 Ohm 
line or not, but it doesn't really matter.  The antenna is designed to be 
set up in an inverted vee configuration.

I do not understand the comment that it was not optimized for NVIS 
performance because that is exactly what it was designed for.  The normal 15 
foot vertical whip normally used is only good for very short ranges or long 
ranges but not for 50-250 mile range.  The only real problem with this 
antenna for NVIS applications is its performance is not very good at the low 
end of the frequency range, 2 to ~4 MHz, because its elements are just not 
long enough.  Other than that it is a fine NVIS antenna.  Note, the normal 
15 foot vertical and short wire antennas also don't work very well at that 
low a frequency.  It is not a good antenna for NVIS with modern ham HF 
transceivers because it seems to have a high SWR.  For the PRC47 it was 
designed to be used with, that is not a problem.  It is not unusual for 
military equipment to be designed for something other than a 50 Ohm 
impedance.  The R-390 for instance has two inputs, one for a high impedance 
whip antenna and one for an ~100 Ohm balanced antenna.  The R-388 (commecial 
model is the Collins 51J3) is designed for a high impedance short whip 
antenna also.   The plots i did show the antenna as designed to be set up, 
provides NVIS coverage.  The SWR plot shows it will not work on the ham 
bands with a transceiver designed for a 50 Ohm antenna.

There is at least two reasons that the hard line coaxial cable is used for 
the center support.  First, it is very convenient.  you have to have some 
kind of support anyway.  The second reason is more important.  Since the 
antenna has a high SWR, greater than 10:1, over at least part of its 
frequency range and it is designed for a reasonably powered radio, ~ 100 W, 
the coaxial feed line has to be able to stand high RF voltage and be low 
loss.  Large hard line coaxial meets those requirements.  RG-58/U does not.

>From the tech manuals for the antenna and the PRC47, it appears that the way 
the transceiver is designed to be used is the box has some short legs to 
hold it off the ground.  The box also has a receptacle on top where the 
(original) vertical slips in.  There is no transmission line between the 
base of the antenna and the transceiver.  The hard line coax/mast for the 
AS2259 appears to be designed to slip into the same receptacle.  The 
transceiver is the base of the antenna.  it appears that there are various 
adapters that can be used with the AS2259 which allow it to be used with 
other equipment.  Perhaps one of these adapters has an RF connector 
compatible with a PL259.  The adapter I saw a photo of seemed to have a Type 
C or Type BNC connector but I'm not sure.

I believe the purpose of the hard line is to provide mechanical support and 
provide a transmission line that will handle power and high SWR.  Yes, when 
a transmission line is not matched, ie, the impedance of the antenna is not 
the same as the impedance of the transmission line, there will be an 
impedance transformation.  I don't believe it matters in this case because 
the length of the transmission line is very short in terms of wavelength and 
the impedance of the transmission line seems to be near 50 Ohms. If you are 
using it to match, you typically would want the transmission line impedance 
to be between the two impedances you're trying to match.  Often the length 
of the matching section is a quarter wavelength.  The length of this line is 
less than a tenth of a wavelength at 80 meters and even less at 2 MHz.  the 
lengths of transmission line that Carolina Windoms and G5RV uses I think is 
~ 50 feet.  A more reasonable length , particularly for 40 meters and 
shorter.

Since there doesn't seem to be a balun at the antenna feedpoint, the 
mast/hardline coax alsmost certainly radiates.  Most people using dipoles 
don''t use a balun and their feedline radiates also.  It just doesn't make 
that much of a difference.  The radiation pattern of an antenna that close 
to the ground is not that clean and you probably don't even notice the 
feedline radiation.  Today, when people use a balun at HF it is almost 
always a couple windings on a ferrite toroid.  These baluns generally work 
well when the impedance is low, 50 Ohms for example, but when the impedance 
is high, 600 Ohms for open wire line, the baluns often don't work well at 
all.  Bottom line, with a wideband antenna like this one i don't think you 
could keep the feedline from radiating no matter what you do.

The resonance frequencies you quoted appear to match the free space 
resonance frequencies of the dipoles.  In an inverted vee configuration very 
close to the ground the resonat frequencies shift.  I expect the resonant 
frequencis of the antenna installed according to instruction will be 
somewhat lower, perhaps just above 6 MHz and just above 9 MHz.  This is one 
of the problems with antennas.  If you purchase one and install it exactly 
as instructed, it may work as expected.  If you change the heights, lengths 
, it is over a different kind of ground, there are conducting objects near 
the antenna such as metal roof or gutters, etc, it may behave quite 
different than expected.  Your comment on the antenna tuner a a couple 
aluminum lawn chairs is exactly right.  If you have a good tuner then you 
can match your transmitter to anything.  Bottom line is at the beginning 
level you want as much metal as high in ther air as you can get it and a way 
of matching it to the transmitter.  The PRC47/AS2259 approach is to control 
the installation and provide a system that can match a wide range of 
impedances.

The AS2259 tech manual is TM-11-5985-379-14&P.  you should be able to find 
it on the internet without much trouble.

Hope this helps.

Yes, there is a lot of information out there and unfortunately a lot of it 
is conflicting.  You get to pick who you believe and spend your money 
accordingly but you have to live with it.  I believe I have the correct 
analysis of the antenna, but who knows.  When it comes to the commercial 
G5RV and Carolina Windom, I wouldn't spend my money for either one but a lot 
of other people do.  When it comes to the AS2259, I would use at as an NVIS 
antenna on 80-30 m and I'd have a good antenna tuner between the transmitter 
and the antenna.

Kerry