You're on the right track, Rich. Compact loops, other electrically small
antennas, and arrays of them will be a growing part of the future for height
and / or space restricted lots, particularly for the low frequency bands.
There is a cost for the size reduction (there's no free lunch). However,
_properly_ done, their only penalty is bandwidth. As you may know, antennas
that are lower to the earth get more DX bang for the same "ground screen
buck" owing to their lower phase center. The possible disadvantage of low
antennas is nearby obstacles.
Beware any loop calculations that take only radiation resistance and
conductor losses into account to predict efficiency. For the 80M loop you
suggest, (a good choice of size, by the way), you should be thinking in
terms of minimum 4" copper or 6" aluminum tube, plus an extensive ground
screen, to get the efficiencies that you state.
I agree with you on avoiding the "all-band" approach to loops, however,
multiple loops, each covering just one octave, will do a quite acceptable
job, and can share the same ground screen. Failure to use a adequate ground
screen will throw away half, or a little more, of what ever efficiency is
achieved by controlling the other losses. Unless, of course, you can get it
3/8 to 1/2 wave above the ground, but heck, if you could go that high, (135
ft.) you wouldn't be worrying about restricted space. A small back yard
(yes, the definition of small is a shrinking number), say 25' by 40', would
allow construction of a good ground for an 80M loop. I better define
"good": On, or just under the surface, all conductors bonded to each other
where they cross, if they do, and conductor spacings of 0.015 wavelength or
less, plus a circumferential conductor bonded to the ends of all other
conductors.
BTW, look at my call on the QRZ web site for a look at a 2" diameter tubing
loop, and a good laugh. The loop is now on 30M. It is "screened" from
lossy earth because it "sees" the close van roof, and not the earth.
Performance is outstanding.
73, Ken
K6HPX
----- Original Message -----
From: "RCARIELLO" <RCARIELLO@si.rr.com>
To: "Towertalk" <Towertalk@contesting.com>
Sent: Monday, September 03, 2001 9:09 AM
Subject: [TowerTalk] What about hams with small lots???
>
>
> Hello to all,
>
> I am happy to see the amount of replies to the "What about hams with small
> lots?" question.
>
> To continue let me add some notes on my personal experiences. As stated
> before the antenna area is my back yard which is 25 feet by 44 feet. The
> bands I enjoy the most are 160-80-40 meters. As it happens I was
introduced
> to an antenna that was used during the Vietnam War. Tried and forgotten by
> Hams because I believe poor construction practices and the lack of the
> needed materials during that time.
>
> The antenna is a Small Magnetic Loop. With today's availability at any
> plumbing supply house for copper pipe this antenna becomes a simple home
> construction project. I am not about to get into the details on antenna
> construction. There are certain things that are a must do and certain
> operating precautions to follow.
>
> The presentation of this antenna has always been for multi band operation.
I
> feel this is the wrong approach for the use of this type of antenna.
Single
> band operation is the key. I have found that the length of pipe should be
> the band in feet. 40-meter band 40 feet of pipe, 80-meter band 80 feet of
> pipe, 160-meter band 160 feet of pipe. Using a square shape the antennas
now
> become 10 x 10 for 40 meters, 20 x 20 for 80 meters, 40 x 40 for 160
meters.
>
> Using 1 inch copper pipe the efficiency becomes, 89% for 40 meters, 88%
for
> 80 meters, 84% for 160 meters. The antennas can be made smaller with the
> same efficiency if a larger pipe diameter is used.
>
> I have found these antennas to work very well for me on all the Ham Bands.
>
> In my first reply I stated my feelings about forgotten antenna designs. I
> use the Small Magnetic Loop as an example. New types of materials become
> available to us each day. What was felt to be impossible in the past now
> becomes possible. I am shore there are many excellent antenna designs just
> waiting to be constructed with the now available materials. I am hoping
this
> reflector will be our means to finding them.
>
> Rich AA2MF
>
>
>
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>
> -----
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> Problems: owner-towertalk@contesting.com
>
>
List Sponsored by AN Wireless: AN Wireless handles Rohn tower systems,
Trylon Titan towers, coax, hardline and more. Also check out our self
supporting towers up to 96 feet for under $1500!! http://www.anwireless.com
-----
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