Monday, November 26, 2007

MFJ 1786 Super Hi-Q Loop Antenna

They say the proof is in the pudding. So here it is.

In the 2005 IARU HF Championship, I operated this antenna and a 80m window-line fed dipole. Both antennas were up about 40' and horizontal. I put them on a switch so I could select whichever was stronger for the signal I was trying to copy. I would estimate that the loop was louder on about 50% of signals, even on 20m. Considering that the theoretical efficiency of the loop is quite a bit lower than the dipole on 20m, I attribute the fact that some signals were stronger on the loop to the fact that the 80m dipole has a very "lobey" radiation pattern when tuned to 20m, whereas the loop has an omnidirectional pattern.

The bandwidth of the antenna when tuned is quite narrow, especially on 20m. This requires frequent retuning. When calling CQ this is a non-issue. When searching and pouncing it is a little bit annoying, but once you get the hang of it you can practically tune it by ear by listening to noise or signal strength, and just tap the tune button as you tune up or down the band. Once you find a frequency to operate on it's easy enough to fine tune it while calling using the cross-needle watt meter. Having the extra work to do with your hand isn't ideal for contesting but if you are using this antenna in the first place it's probably because you are in a portable or antenna restricted situation.

Likes
  • Compact size
  • Works close to ground in vertical orientation
  • Covers 10m through 30m continuously
  • Tunes up quick and easy, especially for small freq changes
  • Can be pre-tuned based on signal strength
  • Narrow bandwidth acts like a pre-selector
  • Omnidirectional radiation pattern
  • Horizontal or vertical polarization depending on installation
  • Aluminum construction is light and strong
Dislikes
  • Requires frequent retuning, even for small freq changes, even for RX only
  • Changing installation height or proximity to metal objects requires fiddling with feeder coil
  • Inaccurate wattmeter
  • Lossy at lower frequencies
  • Must be used in vertical orientation if installed lower than 30' above ground
  • A copper loop would have been higher efficiency
I placed 6th in the Single Op Low Power W/VE category. So there's your pudding. You might not win the contest but you can place top 10. This antenna doesn't suck.

5 comments:

Anonymous said...

Thank you for sharing your experience with this antenna. I for one appreciate that other hams will post their experiences with equipment so that others may benefit. I just bought this antenna and before I could put it up outdoors, winter came to MN. So upstairs on the second floor it sits and running 30 watts, sometimes less, I am back on the air. I get 559 reports and I couldn't be happier.
73 de WB0FDJ

N1YWB said...

I was thinking of selling mine, then my wife decided we were moving to a condo. I'm going to put it on the balcony and make a cover for it and disguise it as a giant ugly flower decoration thing.

I just wish somebody made one that would work well on 40 or 80m. The MFJ one that goes down to 40m is pretty inefficient on 40m. A bigger loop, or a copper loop, would work better. I've heard of guys using similar magnetic loops on 80m but they are either small and lossy or huge and rediculously narrow band, like 3khz. At that bandwidth you can't even touch the knob on the rig without tuning the antenna. Maybe with some sort of computerized controller that automatically adjusted the antenna as you tuned the radio...

Jonathan Jenkins said...

Some useful info there - I'm considering buying one, as I don't have much space, and could maybe take it portable too. Interesting to hear complaints (e.g. on eHam) about retuning alot - I remember having to do that with my old FT200, with valve rf output! Peak it on receive for the noise, and then a bit more with the mic/key down. Sound familiar?
Best wishes,
Jon, G4LJW.

N1YWB said...

Yes, on 20m I find I have to retune whenever I move more than a KHz or two. It becomes second nature though, and I can usually tune it just by peaking the signal strength, and the maybe tweaking it on my first transmit. And if the band is full and I'm searching and pouncing during a contest, then usually just a quick tap or two on the tune button will be enough. On higher bands it's wider bandwidth, and so you tune less often, but still pretty often.

Of course for LISTENING you don't have to tune it nearly as accurately. If the noise is moving your S-meter one unit, then tuning it any more won't help you hear any better. I can scan the whole 20m band for RX, only stopping to adjust it maybe three times.

myles said...

I use a 2 turn magloop, 1 metre in diameter mounted 3 feet above the floor indoors.
NY on 30M is a breeze on WSPR, that's 18,600KM

It is made from 12.7mm dia copper pipe.