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Receive Antennas (below 30MHz) - For all topics related to receive antennas used on HF, MW, LW, etc. For transmit antennas use the Amateur Radio Antennas forum.

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  #81 (permalink)  
Old 06-22-2018, 3:13 AM
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Pattern degradation from un-balanced open loop version:

Quick answer: use a common-mode-choke right after the transformer on your feedline.

EZnec predictions - I ran a horizontal "5th wire" off the loop about 100 feet long to simulate the common mode of a long transmission line. There isn't much common mode actually, BUT there is ENOUGH interaction to skew the omni pattern badly depending on frequency.

So I emulated a common mode choke by putting a 4K load inline with the "5th wire" at the feedpoint, and later at the receiver end. Right after the transformer worked.

Smaller runs of feedline, are accordingly less affected by this, but still - use a choke near the feedpoint.

There are decades long message threads about this on usenet and other boards. So I let EZnec do the talking and am taking it's advice. I'm actually overkill with only 25 feet of feedline, and both the transformer isolator and common mode choke inline right after the 9:1 (5:1 for test) transformer.

Reminder that there *is* a counterpoise if you will with this unbalanced version - the 15 foot long "4th side" of the loop which doesn't connect with the other 45 feet.

Monitoring notes: There seems to be no significant difference between the closed-loop / BALun combo and the open-loop / UN-un combo. I'm hearing all my usual targets, however, I do notice a slight change of direction of the lower angle lobes when I go lower in frequency with the unbalanced open loop.

Long story short - put a choke at the feedpoint.

Ordered the PAR / LNR Precision EF SWL antenna. I'll let you know what happens...

Last edited by hertzian; 06-22-2018 at 3:17 AM..
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  #82 (permalink)  
Old 06-23-2018, 2:24 AM
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Beueller? Anyone out there - this is not my own private thread! Feel free to jump in!

I'm at the limit of my ability with EZnec and empirical comparisons. Anything more requires a professional evaluation.

While waiting for that magic day, the log is easy enough to lay down and see if it suits *your* needs. And like potato chips, you don't only just have to have "one" antenna. They all serve different purposes.

While I have been totally pleased with the log's performance, make no mistake - I take it for what it is, and not for what it isn't, especially when comparing it to other antennas with different purposes - like beverages or bogs that have a totally different pattern / gain / usage scenario. And directly comparing "S" meter signal levels is a total mistake. Don't fall into that trap. Are you hearing things you have never heard before? Or, are you spending more time behind the cans because your ears aren't bleeding for a change?

Don't worry - I'm not about to market this thing and put it into a bag proclaiming that the MKII version now comes with "even more ground loss to reduce pesky noise so you can work the dx like never before !! "

The Log - it is what it is. You either find it useful, or you don't. Simple as that. Just another tool in your toolbox should the need for it arise. My poor-soil condition (think dust bowl) might be different from your lush golf-course lawn. With different results....

Last edited by hertzian; 06-23-2018 at 2:53 AM..
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  #83 (permalink)  
Old 06-26-2018, 5:14 PM
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Normal DX heard ...

I really hate these kinds of "what I heard" reports because a lightbulb under the right conditions can dx.

Just to give some hope to swl's too ...

Best dx on 75m ssb from Los Angeles: Hawaii. Clean. No headphones.

40M ssb - Usual Australians, New Zealanders, and Hawaii. No Rora-Tonga heard tonight. Of course these guys are doing all the work with their big-gun stations. Japanese amateurs coming on strong too.

Early morning east-coast ragchew/dx net on 7.165 +/- LSB provides good testing for directivity. My favored direction is the pacific ocean pointing towards Hawaii / VK / ZL. Yet I can still hear the net control op and others in Florida and the Midwest - albeit just above the noise floor. When I change my feedpoint to an adjacent corner, the east-coast / midwest guys come up stronger, and the Pacific suffers a little bit. These are those two minor lower-lobes and slight side rejection coming into play. But unless you isolate your feedline properly, all bets are off.

RADIO JAPAN: talk about melting my wires! A big thank-you for putting out such a good shortwave signal. Not only is it loud, but the modulation is clean, and yes, fidelity is awesome. A signal to be proud of, and an example for others.

But the point is, that while yes, this kind of loop is primarily a higher-angle omni, thousand-mile or more contacts without headphone-earbleed can happen. At the right time of day, or more properly now, night.

Feedline isolation / choking is critical. Your loop won't tell you if common mode coupling has it pointing straight up in a pencil-like nvis lobe, or off to the middle of the uninhabited ocean somewhere.

Last edited by hertzian; 06-26-2018 at 5:40 PM..
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  #84 (permalink)  
Old 06-26-2018, 6:06 PM
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On-ground loop's closest relative:

The irony of irony's here. The physically horizontal LOG antenna is actually vertically polarized. Yet the closest cousin to this in directional pattern is the physically vertical 1 wavelength loop fed for horizontal polarization. Mind - blown.

This type of vertical loop is understandably not popular for transmit or even rx-only use with it's near-ground loss. Everyone wants those ground-hugging lobes, even if the stations are not able to be heard anyway during the wrong times of propagation or you are sleeping at those times. Going with a vertical polarization feed on this type of loop means you need a good ground. Trust me, I've tried it, and the poor ground, even when I used vertical polarization feed was barely different from my horizontal feed of the close to ground vertical loop.

BUT - look at it this way - it has about 10dbi more signal output than it's on-ground cousin. But that's *IF* you can couple to it properly. Feedline isolation still critical.

While the close-to-ground vertical loop is another project entirely, the pattern is much the same as the LOG, and don't automatically discount it if that's all you can do, as witnessed by my reception of some tame dx above with this usually undesirable pattern.

Last edited by hertzian; 06-26-2018 at 6:39 PM..
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  #85 (permalink)  
Old 06-27-2018, 4:55 AM
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Come to think of it - a slightly different version on a pivot!

Mount the LOG (or dipole on ground too I suppose) onto a lightweight support frame.
During the day, it lays flat horizontally.
At night, pivot it upwards into a vertical position.

Untested by me. Anyone?
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  #86 (permalink)  
Old 06-27-2018, 10:47 PM
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Quote:
Originally Posted by hertzian View Post
Come to think of it - a slightly different version on a pivot!

Mount the LOG (or dipole on ground too I suppose) onto a lightweight support frame.
During the day, it lays flat horizontally.
At night, pivot it upwards into a vertical position.

Untested by me. Anyone?
Wouldn't that change the characteristics, considering the proximity to the ground is part of the design?

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  #87 (permalink)  
Old 06-29-2018, 1:51 AM
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The ground loss seems to have the tendency to turn almost anything (other than a very long bog), into a high angle radiator - and if you hunt around the pattern and isolate the feedline you might find something useful.

I think the best treatise on low down loops is covered by ON4UN's Low Band Dx'ing book. It might be in the ARRL books, or perhaps one of Joe Carr's loop books but I have a feeling it will get short mention. Nobody in their right mind would purposely put an element that close to ground - it seems so obviously bad, but under *some* conditions - if one knows what they are getting into and some traps, it may be useful nevertheless.

Both it and my own EZnec show the same overall pattern between a horizontal log, and a vertical low-down loop - if horizontally fed.

The main characteristic of the in-air vertical loop, with just the bottom leg containing the feedpoint 1 inch above ground, is much less loss of course, but the overall reception pattern is the same.

Electrically, it seems like the ground loss wants to just wrap it's hands around our low down wires, and crumple them up mathematically.
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