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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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Old 06-24-2018, 12:26 AM
Join Date: May 2009
Posts: 2,337
Default Dipole On-Ground formula

Rather than get lost in the loop on ground thread, I thought I'd try to keep it short and under control here. Thanks to KK5JY for sending me on this crazy journey in the first place.

A dipole-on-ground, or "dog", will have a lower resonant frequency than up in the air. Both EZnec and my own ground conditions have revealed this easy to remember formula - which is ballpark, since ground conditions differ at various locations:

365 / f Mhz = half-wave length in feet. Do NOT confuse this with the typical "in the air" formula using 468 !

Matching: For single band use, 50/75 ohm coax is fine, although 75-100 ohms is preferable.

This seems counterintuitive as we know when we bring a horizontal antenna closer to ground, the feedpoint impedance drops. It does but not to zero, -- once you get *below* 1/10th wavelength above ground, the feedpoint impedance starts to rise again, mostly dominated by the ground-loss resistance. Very handy. (Thanks to Les Moxon, G6XN's book HF Antennas For All Locations". (P115, fig 6.17)

As always, try to prevent any pattern distortion, by using a physical / galvanically isolated 1:1 balun, and a nice common-mode-choke at the feedpoint to keep the common mode of the transmission line from distorting it. Or for multiband use, a 9:1 ratio. Preamp? Up to you. Just know that when you start to go higher in frequency than what the dipole is cut for, the pattern distortion generated by the antenna itself is usually undesirable, never mind the problem of keeping the common-mode of the transmission line in check! In my world, I need BOTH galvanic isolation and rf-choking.

(More advanced users will want to try EZnec to model the pattern - maybe they *would* like what they see, like at 4x the frequency the dipole is cut for etc etc)

When I discovered the easy to remember mnemonic value of 365/f mhz = dipole length in feet works well for the on-ground dipole, thought I'd mention it. Insulated wires and connections of course.

Last edited by hertzian; 06-24-2018 at 1:40 AM..
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Old 06-24-2018, 12:33 PM
Join Date: Sep 2012
Posts: 716

How does it work for you, Hertzian?
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  #3 (permalink)  
Old 06-24-2018, 3:43 PM
Join Date: May 2009
Posts: 2,337

The half-wave dipole works about the same as a full-wavelength loop on ground.

Very similar - about 10 to 25dbi down. Pattern (IF properly isolated, and used at or below the design frequency) is the usual higher-angle skywave omni pattern, with lower down bidirectional "lobes" if you will, inline with the wire. Take a perfectly round inflated balloon, and squeeze it with your palms a little bit.

I prefer the loop shape mainly for a space-saving technique, although I had to try this as an alternate for those who might only have a slim side-yard doing nothing.

I use a BAL-UN for the dipole, although if all one has is an UN-UN, then you could "OCFD" it a little bit to make the transformer a bit happier, say a 60/40 split. BUT again repeating myself, one really, really, has to watch out for common-mode coupling with the transmission line, turning it into a different antenna with mostly undesirable reception patterns.

I think the lack of good isolation and the resulting poor reception pattern, more than the immediate ground loss, is what leads to an improper evaluation for many randomly thrown-down-on-the-ground projects. Well, that and expecting to hear dx when conditions aren't right in the first place. Apples-and-oranges comparisons aren't too useful either, like watching the S-meter on your huge above-ground Inverted-L or any other antenna with different characeristics. If I had the space for those, and also a nice low noise floor, of course I'd use that!

If made REALLY long, one might think of it as a center-fed bidirectional beverage on ground. That might come in handy for those guys. For example, at 4 times the resonant frequency, about 2 wavelengths, it shows some desirable characteristics for THAT type of antenna and could be a space-saver if center fed fits on the property better.

I think it is worth a look for those interested in antennas, despite the ground loss. Just make sure your isolation is in place, and used at or below the design frequency, or your strange pattern (usually a totally unusable thin NVIS) will be disappointing.

Last edited by hertzian; 06-24-2018 at 4:19 PM..
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Old 11-05-2018, 7:43 PM
Join Date: May 2009
Posts: 2,337
Default FOLDED dipole better than straight dipole

No surprise here. Just tested out a folded-dipole version, and it is more efficient than a straight dipole.

I used my scrap windowed twin-lead, fed in the center of a 36 foot long scrap, (connected together at the ends to make a folded dipole), and still using the 9:1 isolated transformer, and followed by a common-mode-choke. It had a bit more output than the normal dipole.

I also modeled it as if someone wanted to make a folded version, say 36 feet long and 6 inches to about about 1 foot wide (that's a lot of ground stakes...) and it agrees with what I hear from the twinlead.

So same basic setup 160 meters through 20m is ok. I still use the loop version, but just testing an "F-DOG" for fun. Might come in handy for anyone who finds the dipole version more convenient.
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