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Author Topic: limited space antenna  (Read 612 times)
SV1ENS
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Posts: 124


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« on: December 23, 2008, 11:40:14 AM »

Hi All,
I'm looking for a limited space antenna solution where HF antenna space and size is an issue...
I was looking at the mfj-1775 rotatable dipole and also the mfj-1796 vertical. Both antennas will fit, but how are they compared performance wise ?
Anyone out there in cyberspace with experience with either or both antennas ?

73 & Merry Christmas
Demetre - SV1ENS
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AD5X
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Posts: 1432




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« Reply #1 on: December 23, 2008, 05:22:13 PM »

I have the MFJ-1775.  Convsidering its size, it works pretty well and I've been happy with it for over two years now.  It is unobtrusive and light-weight.  As it is a short, loaded antenna its bandwidth is pretty narrow, especially on 40- and 20-meters.  The only other thing that you need to be aware of is that it does take a while to tune up properly.  Not difficult, but tedious.  I have a review of this antenna in the "Reviews" section of my website at www.ad5x.com.

Phil - AD5X
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KG6WLS
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Posts: 507




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« Reply #2 on: December 26, 2008, 09:38:41 AM »

Demetre,

I’ve had my 1775 up in the air for about a couple of months now. For a compromise, it does very well. Haven’t had any GOOD openings on 10M & 6M yet. I took Phil’s advice on the hardware to keep the spokes in place. Next will be to install a rotor.

73
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G8FXC
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« Reply #3 on: December 26, 2008, 01:22:37 PM »

I've been going through this recently - here's my experiences...

I live in a three storey - a "town house" as they call it here in England. My garden is small - around 10m square - and I try to avoid letting neighbours know that I have a transmitter in the house - experience shows that you get blamed for every single glitch on the TV even when they are not your fault.

I started out with a wire loop running round the eaves of the roof and tuned with an SGC autotuner. It comes out at around 100 feet of wire - I did initially feed it with a length of 300 ohm twin feeder and it behaved pretty much like a G5RV. The SGC gets a decent match on all bands from 160m down to 10m and I've made contacts all round the world on it including both VK and ZL (during the peak of the sunspot cycle). But it does suffer from a fairly high noise floor due to pickup from the house wiring and appliances. It also tends to interfere with TVs, hifi and cordless phones on various frequencies.

I then bought an MFJ Magnetic Loop (the 30m - 10m version) which I mounted horizontally in the loft at around 35 feet above ground level. This outperforms the wire loop by a good two or three S points on all bands that it covers on both receive and transmit. It also has a significantly lower noise floor and causes a lot less interference to televisions and hifi. At just one metre diameter, it's got to be the smallest HF antenna on the market and, being balanced, it doesn't care too much about the quality of your earth connection. The only real downside is the very narrow bandwidth which requires retuning every few tens of kHz.

Most recently I've bought a Sandpiper MV10 on the strength of the good reviews here on eHam. It's a compact, multi-band loaded vertical - Maldol and Diamond have produced similar products. I've got it mounted in my garden, hidden from view by a large tree and with a quarter wave counterpoise for each band. I wasn't expecting miracles, but I did hope that, mounted outside the loft, it would, at least, outperform the tiny MFJ loop. In practice, it performs very much the same as the original wire loop and several S points down on the MFJ loop. On the plus side, it is a lot less prone to noise and the TVI is pretty much non-existent.

In fairness to the Sandpiper, I've only just got it set up and I think its performance can be improved with more counterpoises and some more attention to tuning the elements. But it does have a long way to go before it even equals the performance of the magnetic loop.

For 30m and up, I think that the MFJ Magnetic Loop will be very difficult to beat with anything less than a full sized antenna provided you don't mind the need for frequent re-tuning - it's a one metre diameter loop of thick aluminium tubing and would probably perform even better out in the open. It is quite a lot more expensive than either of the other two options - about twice the price of either, in fact. They do make a 40m version, but that has received very mixed reviews - it really is getting a bit too small for the wavelength.

Magnetic loops are not particularly hard to build - the most difficult bit is sourcing a variable capacitor with a large enough air-gap for the voltages involved. I have an idea to try building a larger loop that covers 40 with reasonable efficiency and tune it with a proper servo motor of the kind used for RC modelling. With a proper control circuit driven from a PC rig control program it would be possible to overcome the need to retune - having the transceiver frequency tracked automatically - possibly a project for the summer...

Martin (G8FXC)
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