TO COUNTERPOISE OR NOT TO COUNTERPOISE, THAT IS THE QUESTION

[W2PFY]

In an effort not to steal an on going thread I pose the following question. I'm putting up a new antenna cut for 1.885. It will be feed with open wire line. I'll be able to get it up about 80 feet on both ends. It will probably sag to 65 feet in the center. Will it help to put a counterpoise under the antenna? Would it help for the counterpoise to have a return path to the antenna tuner?
If this has been covered somewhere, I would like to see it.Any references would be great. One other question, does the feed line need to be at a right angle to the antenna or can it run under the antenna?
 

[WA1GFZ]

I find right angle or close to that works best. A balanced center fed dipole will work better with a good ground under it if it goes out a few wavelengths.

[K8WBL]

I have read about folks laying down several lines or old coax or wire under the antenna to improve ground reflection... I have mine over top of a chain link fence and I do believe it helps.

73, Tim  K8WBL

[flintstone mop]

I dont think that a counterpoise will do anything at 160M. The antenna height you are assembling will be a nice antenna on top band, BUT is still a cloud warmer at best. Possibly 80M or definitely 40M might benefit from a counterpoise.
You would need an antenna like this:

http://www.dxzone.com/cgi-bin/dir/jump2.cgi?ID=19983

AND CLICK ON 160/80M MAMMOTH BEAM


Fred

[k4kyv]

I don't think a ground system or counterpoise has any significant effect on a horizontal dipole antenna, because of the horizontally polarised wave.  For it to have any effect on ground reflection, the radials would have to be many wavelengths long.

I have heard hams say that laying a ground system improved the performance of their dipoles, but I suspect it is more akin to $600 power cords, oxygen-free speaker wire and audio cable break-in: placebo effect.

[AF9J]

I agree.  Counterpoises mainly help when you need an RF ground.  With a  balanced feed antenna, no RF ground is necessary.  IMO ground planes only come in handy for verticals, or the ground plane system is really large compared to the wavelength of the freq. you're operating on, that it helps with ground losses.

[K1JJ]

I've run some structured tests in the past using counterpoises.

At one time I tried to enhance my local signal on 75M. I started with a dipole at about 60' high. I put a 1/2 wave reflector under it at about 8' off the ground.  I added an inductor in the center of this reflector to be able to tune it too.  I also had a remote controlled relay to break the center of the reflector, effectively enabling/disabling it for fast A/B tests.

After lots of listening, tuning, trying it on/off, etc, I found the high angle signals did not change more than 1db at most. It appears to work with computer modeling and sometimes even better as a tuned DIRECTOR that helps enhance the signal into the ground for a reflection... :-)  That's the opposite of what most try to do.

All in all, I wouldn't bother. A better use of the wire is to put up TWO dipoles at different heights. That will give you more signal enhancement as the band changes, than anything else.  Even another dipole at right angles would be a better benefit. 

The idea is to match up the optimum vertical and horizontal angle to present conditions. Dipoles of different heights and at right angles will accomplish this best. Attempting to increase a vertical angle straight up (or nearly so) is wasted energy since most LOCAL signals out 50 or more miles use 50-70 degree angles anyway, depending on time of day, freq, etc.

T

[k4kyv]

Sometime back in the 60's Radio Moscow had mystified monitors worldwide by the consistent strength of their  signal and lack of fading.  It was later revealed that they used some kind of steerable transmitting array from which the angle of radiation could be varied at will, coupled to remote receiving sites located in the target areas which were set up to  determine the optimum angle for the incoming signal.  These transmitted real-time data back to the transmitting site using wired telecommunications links, to optimise the angle of radiation for the circuit at any given instant, sort of a servo system.

[KX5JT]

As I understand, putting a "Counterpoise" under a horizontal antenna creates higher angles of radiation.  It would be like putting a reflector UNDER it so the signal is concentrated upward, i.e. cloudburner.  This is exactly what many NVIS (near vertical incidence skywave) antennas do.

It's not a "counterpoise" since your antenna is balanced.  It's a reflector element!

[KM1H]

I agree with Don and Tom.

In order to get any ground enhancement with a horizontal antenna the reflection point is many wavelengths away.

A single wire under the antenna on the ground or just above it wont do squat. It will be so detuned and de Q'd by the ground as to have no meaningful effect.

Carl
KM1H

[K1JJ]

Hi John,

Yes, a reflector is just what I tried in the tests.  I even went to the trouble of placing an MFJ-259B antenna analyzer at the center of the reflector/counterpoise to manually tune it for about 3700, about 5% longer.   I found that the ground effects had pulled it way lower than the normal 130' length a high reflector would be. It needed to be way shorter to compensate for gnd effects.

Anyway, after tuning it and trying many other inductor reflector settings, I could not really see much difference in performance. I'll say 1db cuz at times I thought I could see something.  I was comparing it against itself with the center of the reflector connection closed or broken with a remote relay.


Another antenna test: I even went to the trouble of stacking a pair of full-size rotary 75M dipoles at 190' and 90', fed 180 degrees out of phase. These were 140' long aluminum elements weighing about 200 pounds each. This should produce a tremendous cloud burner. Well, after all that work, I found a dipole at 60' did exactly the same thing locally.

I also tried a W8JK. (Pair of 1/2 waves spaced 1/8 wave apart fed with openwire) However, the W8JK was stacked with the higher element at 150' and the lower at about 115', shooting radiation straight up and down rather than horizontally. This was a driven cloud burner. But alas, this antenna did no better than a dipole at 60' for local high angle work.

That's why I always say it's hard to beat a simple dipole at 60' for local 75M work....    In contrast, when I erect a high 75M Yagi or pair of loops for low angle 75M DX, they blow away the lower dipole as expected.  The Yagi is often 15-20db weaker locally than the dipole.. no surprises.

Bottom line is I haven't been able to find a high angle cloud burner for 75M that will beat a simple 60' dipole for LOCAL work. Gave up on it long ago.  Makes life simpler to concentrate on low angle stuff anyway.

No doubt the NVIS stuff works, but I have not been able to do so on 75M using the simple techniques as described.

In my case, my conclusion is (assuming these antennas were working correctly) the very high angles are near useless on 75M. Putting energy up there to talk to your neighbor 35 miles away is a waste.  Most QSOs are 100 miles or farther out and using < 70 degrees take off angle.. The overlap of a cloud burner and 60' high dipole may be at about 70 degrees, thus no observed difference.


T

[KX5JT]

Methinks just stretching a full size 75m dipole maybe 10 ft above the ground will give NVIS. Much easier than all those fancy bottom reflectors.

But at anyrate, to answer THE QUESTION... I think the major consensus here is NO COUNTERPOISE required/needed for the antenna in question here.

[K1JJ]

Methinks just stretching a full size 75m dipole maybe 10 ft above the ground will give NVIS. Much easier than all those fancy bottom reflectors.

Tried that too.     When modeling a very low dipole using older MiniNec, the high angle looks great, but it's an error in the modeling program. The later software does a better job at modeling ground and shows poor performance.

But, anyway, when actually testing a dipole low to the ground, like 10-15', on 75M it becomes deaf. The lower, the deafer it becomes compared to a reference dipole at 60' high.  The ground losses and cancellation become overwhelming.

T

[WA1GFZ]

Inverted Vees with the ends close to the ground also don't work.

[Steve - WB3HUZ]

There are some NVIS antennas that will give good gain over a dipole. But they are large. See the two plots below for the Shirley and Jamaica antennas compared to a dipole.

[W2PFY]

Steve, where can I see a picture of the antennas you mentioned?

[K1JJ]

Here's some info on these ants.  There's more when doing a Google search for these names.  (Shirley or Jamiaca NVIS)
http://www.vmarsmanuals.co.uk/newsletter_articles/nvis.pdf

Check out picture 27 and 28 for diagrams of the two antennas:
http://www.authorstream.com/presentation/Davidino-50078-NVIS-leq-Ordinary-Propagation-NVISPropagation-Using-NVISsuccessfully-as-Education-ppt-powerpoint/



Hey, those are pretty cool designs!  It appears the key is mounting a pair of open wire fed, two-half-waves-in-phase, spaced 1/2 wavelength apart between 1/8 and 1/4 wavelength high. (Jamaica)  Or doing the same thing with a pair of 1/2 wave dipoles. (Shirley)   They both include that critical 60-70 degree take-off angle on 75M for local work.

This is similar to the "Pair of phased dipoles" some of the guys use when run in the "in-phase" position.

I'm changing around antennas now and think I'll try one of these Shirley NVIS systems.  Now that I see the modeling supports it, it's worth another try.  In the past I never tried the 1/2 wave spacing. I'll design it for 3850 and mount it about 45' high, right in the middle of the suggested height range.


BTW, Steve, could you show the horizontal plot?  They say it is omni-directional. If so, then it would not be critical to mount it in any particular orientation - this wud help make finding supports easier.  The 1/2 wave spacing makes simply putting up a pair of openwire-constructed dipoles 1/2 wave apart and fed with the SAME open wire type, and connected in phase would do the job for a Shirley.  The similar (openwire and dipole) construction provides a perfect match and zero phase power transfer. I will match the end of these openwire feeders with a stub to coax for the run to the shack.

The Jamica wud take up much more real estate but give a slight more gain..

Thanks for the info, OM.

T

[Steve - WB3HUZ]

Dunno, but I'll describe them.

The Shirley is two dipolesfed in phase and spaced 0.65 wavelenths apart. The height should be no greater than 0.25 wavelength. In my model, the greatest gain was at a height of 60-65 feet.

The Jamaica is the same as the Shirley, except driven elements are a full wavelength length (or in other words, a pair of half-wavelengths in phase). The height that produced the greatest gain at 90 degrees elevation was 50 feet. As you can see, two little lobes at the lower angles are produced at this height. IF the spacing is changed to 0.5 wavelength and a a height of 60-65 feet is used, there is less gain at 90 degrees elevation, but the low angle lobes are gone.

[Steve - WB3HUZ]

I'm pretty sure the azimuth pattern omnidirectional. I'll double check tonight.

More info on NVS and the two antennas at the link below.


www.rientola.fi/oh3ag/garec/documents/NVIS.leq.ppt

[K1JJ]

S,
It's interesting that the Shirley's vertical cross over point compared to a reference dipole is at about 65 degrees. So this may be why my NVIS experiments didn't show much results cuz 65 degrees and lower is widely used on 75M for local/medium work.  It would still be interesting to try for close-in strapping.

The difference between the high DX loops and the Shirley should be 15-20db depending on angles. Any time there is a difference like that due to vertical angles, it's a useful complement of antennas to cover a wide range of band situations.

Looking around the field today, I can see it's not an easy thing to put up with the wide 1/2 wave spacing and still keep interactions with other antennas low.  This is something that might work well out in the woods, since the height required is normal tree height.

T

[Steve - WB3HUZ]

Looks like they are both mostly omni at the higher angles but the Jamaica is more so.

[K1JJ]

That's an encouraging horizontal pattern. Looks like a turnstile pattern.

What do you see for input impedances on the Shirley at 30',  45' and 60'  high?  And, you're using .625 wave spacing, not .5, right?

  I think two separate coax feeds with the coax ends tied together and matched would work FB.  I'm hoping the dipoles are so far apart that the mutual coupling will be low and not screw up the phasing as much as the close spaced stuff we usually work with.

T

[Steve - WB3HUZ]

The Shirley is spaced 0.65 WL (168 feet). The FP Z for both dipoles is as follows.

30': 47.4 + j2.9
45': 59.2 + j0.5
60': 72.9 - j11.9

See elevations patterns at those respective heights compared to a dipole at the same height. The greatest gain is at 45 feet, but the difference in gain compared to the dipole with within a fraction of a dB for all three.

[K1JJ]

Looks good, Huz.

Either 50 ohm or 75 ohm coax should do FB.  The paralleled feeders can be matched easily with a HB 1/12 wavelength coaxial balun to a standard 50 ohm coax run to the shack.

I'm looking around for the right spot to erect it. 45' sounds like a good height, though it's cool it works so well at 30' high.

Tnx for the plots.


BTW, Terry, there ya go. Probably more than you wanted to know, but with all that room you have, you could easily erect one too. Amaze your local friends!

T