Begin forwarded message:
From: R. Measures <r@somis.org>
Date: December 26, 2004 8:44:36 AM PST
To: Joe Isabella <n3ji@yahoo.com>
Subject: Re: [Amps] OFF TOPIC: Loop antenna question
It is a half wave dipole on 80m, Joe, but it's unbalanced voltage-fed
at one end against ground instead of balanced current-fed at the
center. Since the ends of a half-wave are high-Z, a Hertz is usually
matched with a coax-fed L-network at ground-level, Thus, a Hertz is
typically an inverted-V configuration at the feed-end -- with the
center and far end elevated above ground, On 160m. it's a quarter-wave
end-fed, a,k.a. a Marconi. Typical feed R on 80m is c. 2000-ohms, and
on 160m it's c. 20-ohms. /// If such an antenna were center-fed, the
results would be similar on 80m - but shockingly worse on 160m due to
the exponentially lower feed-R/radiation-resistance and through the
roof XC. However - as an end-fed, the antenna is resonant on both 80m
and 160m, so there is no troublesome reactance to have to deal with on
160m.
-- notes - A Hertz antenna can be erected in "absolutely, positively
no antennas" areas since it is virtually invisible if made with #30ga
Cu (or, preferably, phosphor-bronze) and held up at the center and far
end with halyards fastened to trees. In warm climates, the Mexican
fan-palm (washingtonia robusta) is ideal for this purpose. Mexican
fan-palms are quite common in Beverly Hills, Santa Monica, and in my
yard. The 1-gal. size goes for c. 10-bucks at Green Thumb
International®. Mexican fan palms can grow to a height of 30m. They
periodically need to have their dead fronds trimmed as they gain
altitude. My current, voltage-fed antenna is held up by two,
>36-footers with halyards. An alternate feed method is to elevate the
L-network above ground, using an elevated quarter-wave counterpoise for
an artifical ground. In my experiences, Hertz antennas need a pre
L-network coax ugly-balun to keep RF from getting back into the radio
and amplifier.
cheerz
On Dec 26, 2004, at 6:20 AM, Joe Isabella wrote:
I thought a Hertz was a dipole...
"R.Measures" <r@somis.org> wrote:
On Dec 26, 2004, at 12:40 AM, Alek Petkovic wrote:
Have you thought of a section of open wire line in the feedline at
the
feed point which puts the equivalent amount of wire into the loop to
resonate it on 75m?
eg: a 75m loop for 3.9MHz is around 258' and for 7.2 MHz it would be
140'. That is a difference of 118'. If you were to use a 59' section
of open wire, it would put 118' of extra wire into the loop.
The wire in a balanced feedline does Not add to the wire in the
antenna
-- i. e., antennas and feedlines are not like series resistors. .
The best antenna I have used for limited space on 80m - 160m is the
(end-fed against gnd) Hertz. A 100' by 60' lot will accommodate a
fairly efficient Hertz antenna. The tradeoff is that a
remote-controlled L-network is required at ground level at the driven
end of the wire.
If it is not possible to do it that way, you could perhaps put one or
more loading stubs, say at the support points, still using open wire,
to make the antenna up to the total required length.
I have not tried this but it could be a cheap solution and well worth
some experimentation.
Good luck,
Alek. VK6APK
ps:
I hope to be making a nuisance of myself on 75m over the next week
from my beach house using my 2 x 3-500z and Carolina Windom 80. Watch
out for me. I'm heading down there tomorrow.
At 12:53 PM 26/12/2004, JimMoci@aol.com wrote:
Happy holidays to all on the amps reflector. I have an off topic
question
about a loop antenna so if it is not appropriate please remove it or
respond to
me directly via email. I am asking here because there are so many
experienced
and knowledgeable people on this reflector and the info I receive
will be very
useful.
My situation: I currently have a full sized 40 meter delta loop @ 30
ft and
it is horizontal to the ground. I use it on 40 thru 10 with good
results. The
antenna exhibits a good SWR on 40 and an SWR less than 2.5:1 on 20
thru 10
meters so the internal auto antenna tuner in my Icom solid state amp
can handle
the mismatch. What I would like to do is use it on 75 meters but
dont
have the
room for a full sized 75mtr loop. The loop is to small for 75 and
has
an very
high SWR, to much for the internal tuner to deal with. So far I have
tried
splitting it in the middle, essentially a full sized 75 meter dipole
folded back
on itself. It works ok, but not a stellar performer. It also lost
the
advantage
of being a quiet receiving antenna, a characteristic loops are known
for.
I have a couple ideas that I want to try but figured I would ask
before I go
climbing tress out in the cold. The first idea was to add another
full turn to
the loop, separated by a few inches. Essentially it would be a 2
turn
loop,
totalling about 260 feet, but only taking up the space of a 40 meter
loop. The
other idea was to add a large inductor in the middle of the loop. I
have a
large, silver plated, flat wound inductor from a broadcast
transmitter that would
do nicely. I also have a vacuum relay that I could use to short it
out for 40
thru 10 mtrs.
My main question is will the two turn loop work? If so will it be
better than
the inductor?
I realize that I could use a large roller inductor tuner in the
shack
but
that defeats the purpose of my lazy hams push button station. My
solid state
kilowatt is silent and automatic and it is my goal to keep the
station totally
push button.
Thanks for any and all advice- It will save me many trips up the
tree
and
there is snow in the forecast. Why didnt I do this in the
summer?!?!?!
73,
Jim W3ATV
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Richard L. Measures, AG6K, 805.386.3734. www.somis.org
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