[Amps] OFF TOPIC: Loop antenna question
R.Measures
r at somis.org
Sun Dec 26 11:58:57 EST 2004
Begin forwarded message:
> From: R. Measures <r at somis.org>
> Date: December 26, 2004 8:44:36 AM PST
> To: Joe Isabella <n3ji at 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 at 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 at 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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>>>
>>> http://www.qrz.com/vk6apk
>>> http://profiles.yahoo.com/vk6apk
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>>>
>>>
>>
>> Richard L. Measures, AG6K, 805.386.3734. www.somis.org
>>
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>
> Richard L. Measures, AG6K, 805.386.3734. www.somis.org
>
>
Richard L. Measures, AG6K, 805.386.3734. www.somis.org
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