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Ground radial under horizontal doublet or dipole?




 
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Author Topic: Ground radial under horizontal doublet or dipole?  (Read 13118 times)
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ke7trp
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« on: November 15, 2010, 11:37:35 PM »

The other night, I was reading a 1940s CQ magazine and found an article about placing ground radials under flat top dipole antennas.  This article claimed that if you live in areas of the country that had poor soil conductivity, this would really help the antenna function. After this, I read a more recent article in another book saying this was hogwash and said it was pointless, confused, I asked around a bit and the consensus was that it would help ONLY shoot more signal up so it would help local or close in contacts and would hurt DX.

Can we have a discussion about this please?

The article did not explain how to hook this radial or radials up but I would guess, Its just a tuned length of wire under the dipole staked into the ground at each end.  Or, would you hook this radial to the shack ground or tuner ground and then run it out under the dipole?

C
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KA2DZT
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« Reply #1 on: November 16, 2010, 12:59:36 AM »

Probably not connect the ground radials to anything.  I think ground wires under a dipole would improve ground reflections just the way wet terrain would under any antenna.

Unlike a vertical that needs a ground plane to complete the 1/2 wave antenna, a dipole already is a 1/2 wavelength.

Just some quick thoughts about your post.  I sure you'll get more theories about this concept.

My FW loop is up over a stream and nearby wet terrain which I think helps it work well.

Fred
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k4kyv
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Don
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« Reply #2 on: November 16, 2010, 03:29:34 AM »

I don't think a single ground radial would have any noticeable effect at all unless your  soil conductivity was so poor that it was equivalent to a  good insulator, then the radial might act like a reflector or director to either beam your signal up vertically or produce a vertical null if it happened to be the exact appropriate length.  To enhance your ground reflections in the real world, you would need a radial field several wavelengths in diameter.  IMO it would be a total waste of time.
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Don, K4KYV                                       AMI#5
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« Reply #3 on: November 16, 2010, 08:37:15 AM »

Clark,
          You're getting close to describing a NVIS (Near Vertical Incidence Skywave). Read Jerry Barry's article in the Nov.'05 issue of ER. Very interesting. I have a wooded area toward the back of my lot and I'd like to try one for 75M.

73, Phil 
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Steve - WB3HUZ
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« Reply #4 on: November 16, 2010, 09:40:49 AM »

A single one is completely useless. Many may make some difference at the higher angles. But unless the radials/ground screen extends wavelengths from the antenna, there will be NO difference at the lower angles.

I know this was discussed in a previously. I did some modeling and posted the results. Do a search.

But in short, I wouldn't bother with radials under a horizontal antenna.
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W9GT
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« Reply #5 on: November 16, 2010, 09:49:28 AM »

Steve, et al,

Didn't they used to call that a counterpoise?  I recall that MFJ and others even sold tuners to establish an "artificial ground" by using a tuned circuit connected to a ground wire or radial.  Maybe only effective under a "short" antenna?  Never tried one, don't know if there is anything to it.

73,  Jack, W9GT
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« Reply #6 on: November 16, 2010, 10:49:32 AM »

Some years ago I put a bunch of hours into "Terrain Analyzer" (TA) by Brian Beezley. It did a good job of modeling horizontal antennas over real ground going out for miles if you desired - using ground gradient data that you input from a detailed local hiking map. It also used an estimate of the Earth soil.

I learned some interesting things. First, the best "local" high-angle location would be a moon crater that sloped up about 45 degrees on all sides. This would form a nice dish-type reflector for higher angles around ~50-70 degrees, useful for local to medium distance work only. The low angles were non-existent with this config. That's why some 75M guys in a big hole are loud locally but PW far away.

I also found that to enhance your low angles, you would need to have an excavator custom form your terrain for a mile or more around the QTH. I was able to get another 6DB gain! from a simple 20M Yagi by doing so. To do this you need almost perfect Earth ground and need to adjust the reflections out for a mile or so until the vertical lobe forms almost a spiked point...  Grin  But if the frequency changed much or the height of the Yagi changed, the gain went to hell and became more random from lobe additions and nulls, etc.

The bottom line is radials are FB for verticals that demand them for operation, but for horizontal antennas, you would need radials put out for thousands of feet, and then in the perfect gradients to enhance a signal . Pure fantasy.

That said, I would think based on this software, assuming it was accurate, that VERY high angle enhancement could be done with radials below the antenna for horizotals. Just like NVIS, but these angles are pretty useless for ham use on 75M ,etc. Angles of between 50-70 degrees or so are used for local work for 100 miles and more away. It varies, of course, depending on time of day, seasons, etc - but the angles above 70 degrees, that are enhanced by radials under the  antenna are useless for operation.   Bottom line is to put the effort into the antenna supports, gain elements and matching to get a good antenna installation. Horizontals are at the mercy of your existing Earth terrain and there's not much you can do about that.

T
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ke7trp
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« Reply #7 on: November 16, 2010, 10:50:52 AM »

Ok sounds good guys.  We have sand here. Its Dry as a bone. Then 6 inches below that we have solid rock. We are installing a new fence now and this is a real bear to dig holes in.  The thought of better performance using a few wires strung out on the ground interested me. However,I think its going to just do nothing, or it will beam the signal up.  I already have a BIG signal for close in.

C
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K5UJ
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« Reply #8 on: November 16, 2010, 01:05:06 PM »

Steve, et al,

Didn't they used to call that a counterpoise?  I recall that MFJ and others even sold tuners to establish an "artificial ground" by using a tuned circuit connected to a ground wire or radial.  Maybe only effective under a "short" antenna?  Never tried one, don't know if there is anything to it.

73,  Jack, W9GT

Jack, i used to own one of those MFJ artificial ground tuners.  It is an inexpensively constructed matching network designed to make a 1/4 w. wire have low inductive reactance on all bands.  The idea is if you have a shack in a location far removed from a short wide strap run to real earth ground rods, you can hook a wire up to this thing, throw it on the floor, hook the MFJ box to your rig's ground lug and it will make the wire be at ground potential so the rig acts like it is grounded.  It turned out to be not well constructed, I could not get it to work right, and as my shack was in the basement, I moved it to a wall and real earth ground and sold the MFJ box. 

Rob 
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Ralph W3GL
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« Reply #9 on: November 16, 2010, 01:26:12 PM »

    Okay the conscious is a counter poise under a .5 lambda wire
    (high or low) is a waste of material and time but no one looked at this:

     Take a wire 3 to 4 % longer (actually 5% would be better) than the half
     wave and put it .15 or .2 lambda below the 1/2 wave, or off to either
     side by 20 or so degrees.

      Model that and you are no longer wasting time,,, It WILL make a change
      in the take off angle.   I did this several times in the field to improve
      a field com circuit back to HQ while on active duty...

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ke7trp
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« Reply #10 on: November 16, 2010, 01:44:47 PM »

Lets see some modeling guys!


The MFJ artificial ground has one pupose in life.  Its used for a guy that has his station on the second floor and does not have good grounds.  My buddy had this scenario and killed his internet, phones ect with RFI in the shack.  He layed out tuned wires but then realized that those tuned counterpoise wires where good for one band. He purchased the MFJ product and was able to tune the counterpoise perfectly.  Now, He has NO RFI in the shack at all.  He made a chart and when he flips bands, he just turns the knobs.  This made no difference on his transmit or RX that we could tell.

C
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KB2WIG
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« Reply #11 on: November 16, 2010, 02:18:59 PM »

Drifting somewhatt farther off topic. The Johnson Ranager manuel has a nice section on correcting ground problems. Check out BAMA fer yer copy.


klc
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k4kyv
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« Reply #12 on: November 16, 2010, 02:41:56 PM »

Sometime you only need counterpoise wires for one band. I had a severe 40m rf feedback problem.  Rf would get in the audio, and some of the random wiring in the shack was so hot that a neon lamp would light up when I held it near.  I tried reconfiguring the hook-up and various grounding schemes but nothing seemed to help. Finally, I tacked some quarter-wave counterpoise wires under the floor joists of the shack, and terminated them at a grounding point near the transmitter.  That cured the rf problem. The wires will light up a neon lamp at the far end, but the shack wiring is no longer hot and the feedback is long gone.  This problem had never existed on 80 or 160, so the one-band solution was perfectly satisfactory, and I'm not bothered with having one more thing to tune up when changing bands.
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« Reply #13 on: November 16, 2010, 08:12:47 PM »

maybe my metal building (shack) will be good for something after all.
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Steve - WB3HUZ
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« Reply #14 on: November 16, 2010, 09:56:04 PM »

Quote
It WILL make a change in the take off angle.

If it makes any change it will be small and at only one or a very small range of take-off angles. The effect of the nearby earth just swamps out any effect of a single wire, no matter the length. Now if all the wires are 0.25 to 0.5 WL above the ground, things can be different. TANSTAAFL.
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Steve - WB3HUZ
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« Reply #15 on: November 16, 2010, 10:04:42 PM »

http://amfone.net/Amforum/index.php?topic=24943.msg186747#msg186747
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