re:MFJ-1786 Hi-Q Loop

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From: david (d.e.) baker (debaker@bnr.ca)
Date: Mon Jan 10 1994 - 12:29:00 EST


In message "MFJ-1786 Hi-Q Loop", you write:

>I saw an ad in one of the radio mags over here in the UK about the
>MFJ-1786 Hi-Q Loop aerial (sorry antanna). I was wondering if anyone had
>tried this type of aerial out for QRP work, and if so how successful they
>found them.
>
>Thanks, 73 Martin
>--
>+-------- Email ----------------+-------------- Radio -----------------+
>| qrp@chalford.demon.co.uk | g4dul@g4dul.ampr.org [44.131.5.145] |
>| coburn@gec-mi-at.co.uk | NTS BBS network G4DUL @ GB7KHW |
>+===============================^======================================+

Martin,

I have used the MFJ loop, and it does indeed work. It will outperform most
any dipole that is not at least a half wavelength up in the air. I just
got an AEA ISOLOOP which is about the same thing as the MFJ, and it
usually gives me 2 to 4 S units over my 40 meter dipole (on any freq.
between 10-30, of course). For comparison, my 40 meter dipole is attic
mounted, about 15 feet above ground level, in a U shape. The loop is
also horizontal, at 20ft, still inside the attic... However,before I
mounted the loop, I used it mounted on a 5 ft pole, right in the room
with me, and it still beat or equaled the dipole's gain. The major problem
with a loop is tuning. They are slow and difficult to tune, due to their
high Q nature. Bandwidth is also small due to the Q, typically 10 to 100KHz
depending on freq. This will also help eliminate any TVI, especially at QRP
levels. :)

Homebrew QRP loops should prove to be very functional, the only problem
will be that it would be very difficult to make one with a remote tuner.
As easy project might be a 40Meter QRP loop. Using loop antenna construction
guidelines, it would be simple to create a loop with a fixed capacitor, but
then it would only operate on a fixed frequency and at low power only.
The other big problem with loops is that they create very high voltages
on the loop during transmit. Commercial loops like MFJ, AEA, or AMA's are
designed for higher power, and thus require HUGE tuning capacitors with
high voltage ratings.

Bottom line: if you can't mount an antenna (dipole) up at the ideal height,
             a loop will probably beat it. I would still recommend putting
             up a long wire or something similiar just for scanning. It is
             somewhat tiring to try to retune the loop as you scan the band.

Hope this helps. Check the November 1993 issue of QST or one of the ARRL
antenna handbooks if you want some more background info.

______________________________________________________________________
| David E. Baker | Internet: debaker@bnr.ca (Richardson, TX, USA) |
| IP: 47.122.65.7 | UnixID: crchh7b0 | Bell-Northern Research, Inc. |
| Callsign: AB5PI | Packet: AB5PI@N5AUX.#DFW.TX.USA.NA | Smile! ;-) |
| My opinions do not necessarily reflect the opinions of my employer |
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