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1-10 of 10 messages
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Page 1 of 1
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Counterpoise / radials question(s)
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by KC8TMV on November 6, 2009
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I want to put counterpoise wires directly underneeth my off center fed dipole.
Here are my questions:
1. should I put one ground rod directly under the feed point with the counterpoise wires(radials) going in both directions?
2. Should that ground rod be attached to my Single Point Ground at the shack?
3. If "yes" to #2, how often do I need ground rods attached to the ground wire going back to the shack? It is about 100' back to the shack from the directly under the feed point. My thinking here is that if there are no ground rods in between, this wire will not be a gound, but will end up being another counterpoise or end up being an antenna!
I could be wrong on all points here. I am not an expert or I would be giving the advise, not asking for it!
I am trying to learn. -Steve
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RE: Counterpoise / radials question(s)
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by K5LXP on November 6, 2009
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> I want to put counterpoise wires directly underneeth
> my off center fed dipole.
We can start with that- why?
Mark K5LXP
Albuquerque, NM
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RE: Counterpoise / radials question(s)
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by K0BG on November 6, 2009
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If you're trying to quell common mode current, you're in for a surprise. Considering all, common mode current is a feature of all off-center-fed dipoles, no matter their configuration. Is that what you're trying to do?
Alan, KØBG
www.k0bg.com
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RE: Counterpoise / radials question(s)
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by K0ZN on November 6, 2009
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Hi, Steve.
Short version: there is almost no point in doing that.
Horizontal antennas don't need radial or ground systems, except for lightning protection. You will gain little to nothing for your efforts. A ground mounted vertical is a completely different story because the ground is the other HALF of the antenna....not so with a horizontal antenna.
The best thing you can do with an OCF antenna is to make sure you put an isolation choke or some other method/device to keep the currents on the shield from coming back into your shack/rig. Off Center Fed antennas are NOT balanced and have a high potential for RF in the shack problems.
It sounds like you are interested in antennas and building them. BY FAR...the best investment you can make in your station at this point in time.... istime put in doing a little reading and studying in a good antenna book like the ARRL Antenna Book. Antennas are not complex, but there are certain parameters that must be met and they are also not necessarily intuitive. Unfortunately, there is ton of myth out there and the best way to get around that is with a good book and some study time. Antenna knowledge is not something you can pick up in a few blurbs on an internet site. The efforts you put in to really learn and understand antennas will reward you with a stronger signal, more contacts, less frustration and more enjoyment of the hobby.
Good luck & 73, K0ZN
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RE: Counterpoise / radials question(s)
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by KC8TMV on November 6, 2009
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Why: 1. lower noise level 2. better tuning 3. remove RFI in the shack (common mode current) issue 4. best radiation pattern possible (height 70').
So a counterpoise is not the answer. There is a lot of colflicting information out there, including counterpoise examples from different M.A.R.S. sites.
I have a common mode choke in line, but that does not seem to help the RFI issue much.
You are right that reading is a good thing, but sometimes knowing what to ready is more important. Most of the ARRL books cover so much that it is a little overwhelming!
-Steve
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RE: Counterpoise / radials question(s)
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by AA4PB on November 6, 2009
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Probably the best thing you could do for the common mode current problem is to center feed the dipole. OCF antennas can provide a workable match on multiple bands but at the cost of common mode current on the feedline.
If your antenna is located 100-feet away from the shack then your shouldn't have any direct coupling problems. However, if one end of the antenna is near the house (i.e. the antenna runs from the house to a tree and the feed line runs parallel to the antenna out to the feed point) then you probably do have some direct coupling issues. If so, the only solution will be to re-orient the antenna.
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RE: Counterpoise / radials question(s)
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by K0BG on November 6, 2009
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Part of the problem is Steve, a lot of information that's out on the net is based on flawed data, and junk science. Just because you read about it on a military site, doesn't mean squat!
With respect to the OCF dipole (sort of a misnomer actually), the original was fed with a single wire. Sort of a top loaded vertical, which was (supposedly) easier to match. It wasn't, but that's another story. That antenna, did indeed, require a radial field. Not a counterpoise, by the way, as that's a whole different animal often confused with ground planes and radial fields.
After Mr. Windom, W8GZ (sk) came up with the first published info on the OCF, several others came up with variations both coax and ladder line fed. Technically, they had little in common with the first OCF antenna. But, alas, the Internet has fuzzed those concepts all out of proportion.
The truth is, it is darn hard to beat a simple, center fed, dipole. Set up a series of them, properly spaced, and wallah! You have a beam!
Alan, KØBG
www.k0bg.com
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RE: Counterpoise / radials question(s)
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by N5LRZ on November 6, 2009
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Dipoles, OCF or Centerfed do not need counterpoises.
I strongly urge you to get a fairly new copy of the ARRL Handbook and the ARRL Antenna Book and study the sections on antennas and antenna theory.
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RE: Counterpoise / radials question(s)
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by WB2WIK on November 6, 2009
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If you lay out zillions of ground radials, or even a ground screen, all over your yard, you can improve ground conductivity below your antenna which actually might improve performance to some degree.
However, those would really have to be big, long, and run for hundreds of feet in all directions. It's a lot of work for not a lot of gain.
But to reduce RFI in the shack and stuff, usually all you need is a line isolator installed just outside the shack, in the feedline. Several are on the market. Most of these do two things: They choke the common mode current by adding a pile of ferrite around the line's outer conductor, and then they also ground the shield of the coax to ground (you install the isolator right on an 8' ground rod) to short common mode current to earth.
90% of the time, this gets rid of the problem unless it's caused by direct radiation from the antenna into stuff in the house. But if your antenna is up 70' above ground, that probably isn't happening.
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RE: Counterpoise / radials question(s)
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by WB6BYU on November 6, 2009
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There are cases, especially with low dipoles over poor
ground, when adding one or more reflector wires to
improve the ground may help to improve your signal
slightly. An extensive analysis by W4RNL is here:
http://www.cebik.com/content/a10/wire/n3.pdf
(Yes, you have to register to read his site, but there is
a wealth of information there. It is worth a few days of
reading, and coming back to when you have questions.)
Basically the wire is acting as a reflector of a 2-element
yagi. The earth does this, too, and generally makes a
pretty good reflector. That's why adding more wires
doesn't help the signal much. If you have very poor
ground it can add about 1dB to your signal, depending on
the antenna and reflector heights. Just as with any
2-element yagi, adjusting the reflector for maximum
gain lowers the radiation resistance of the antenna
and narrows the SWR bandwidth. When your antenna is
a quarter wavelength above ground, the spacing is
enough that adding one or more ground wires doesn't help
much. Even extending it to a ground screen one wavelength
square doesn't improve your signal as much as 3dB under
optimal conditions (certainly not much for all the work
involved.) Over average ground the improvements are
less, as the earth itself provides an adequate reflector
without any additional help. (And note that the reflector
wires generally are NOT connected to ground.)
So, there are situations when a reflector under your
wire might improve the radiated signal slightly, but
in most cases it isn't worth the effort.
There are a couple cases, however, when it might be
more of an improvement. For example, on 40m your antenna
is close to 1/2 wavelength high, and the ground reflection
will actually cancel the antenna radiation at high
angles. Adding a reflector wire around 1/8 wave length
below the antenna would improve your signal on that
band.
Well, it would if you were using a dipole. One of the
problems with using an 80m OCFD for 40m NVIS is that
there is a null in the pattern at high angles. You
might find that adding a 40m reflector around 15' below
ONE END of the OCFD will make an improvement on that
band - I'll have to model that and see.
But as far as the other problems, adding reflectors,
radial wires, or any sort of counterpoise on the ground
won't help. If you have RF in the shack (which the
OCFD is known for due to the unbalanced feed) you need
a better balun at the feedpoint and/or better RF chokes
at either end of the feedline. (And perhaps another
choke in the middle if it happens to be a resonant
length.) Grounding the feedline at some point, like just
before it enters the house, may also help. Reducing
the common mode current may also improve the noise level
if it is due to pickup on the feedline from devices in
the house. If the problem is a noisy power pole or
other souces that are being picked up by the antenna
rather than the feedline, the only solution is to
eliminate the noise sources themselves.
Unfortunately the history of the introduction of NVIS
to the military has included a number of errors that
still persist. Not that the mode isn't useful - hams
have been using low dipoles for local ragchews for
years - but some of the standard practices may not be
based on firm facts, and some may not be as applicable
to ham usage as they might be to military field units.
One significant error was due to the use of MININEC for
computer modeling of low horizontal antennas. The ground
model used has a number of known problems which tend to
overstate the ground reflection gain. This made it
appear that very low antenna heights gave high gain,
when in practice the ground losses reduce the efficiency
of antennas significantly below about 0.1 wavelength or
so. Yes, you can make contacts on 80m using a dipole
that is 6' or 6" above the ground, but your radiated
signal will be stronger at 30' high.
Military field units are also worried about ground wave
radiation for two reasons: because it is prone to DF
by enemy forces, and because the ground and sky waves
may be received together and cancel each other at nearby
stations. Ground wave signals (due to a vertically polarized
component to the antenna) are normally more of an issue
when using an end-fed wire connected to a packset rather
than a balanced dipole (though feedline radiation can
contribute to the problem.) Being subject to DF is
normally not an issue for ham use, while stations that
are close enough for the two transmission modes to interfere
with each other are probably within range for 2m FM simplex,
so wouldn't be relying on HF.
That's not to say there isn't a lot of good information
out there, too, but you need to sort though and see what
makes sense. Sometimes someone will take a good idea
and extrapolate it beyond the point where it applies:
I've seen a number of MARS or CAP stations with
reflector wires under their antennas even when the
local ground is pretty good. (It can't hurt, and
most stations don't know the quality of their ground
anyway.) I've also seen recommendations for using a
random wire with a reflector below it that is 5% longer:
that doesn't help since the reflector length is a function
of the operating frequency, not the radiator length.
Unfortunately there are a number of "recommended" NVIS
antennas that are far worse than necessary due to the
inappropriate application of sound principles.
Good luck!
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