counterpoise antenna

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counterpoise antenna

Postby lhuchison » Thu Dec 03, 2015 11:26 pm

I'm running an outside 10 metre end fed antenna running into a balun in my loft. This is for receive only. I can't run an earth from the balun as its too high in my loft to get to an earth stake, which i know is ideal.
So, i measured a 10 metre length of wire which i connected to the earth terminal on the balun and run this wire inside my loft space.
Am i right in thinking this makes a counterpoise for the 10 metre outside antenna or have i turned it into a dipole? half in half out? It seems to improve things, noise levels etc, and sensitivity .
I have a basic ( T- match) antenna tuner on order which i hope will improve things.? Any advice welcomed. :smiles:
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Re: counterpoise antenna

Postby G4RMT » Fri Dec 04, 2015 12:40 am

Well you have really turned it into a dipole, from the point of view of loading and impedance, but the radiation pattern will be dependent on the combination of the two, and how they interact. A dipole, when setup conventionally has hardly any sensitivity in the direction of the dipole elements, so the counterpoise really acts as a somewhat radome direction dipole element - so the direction of almost no signal will not be so pronounced. Used horizontally, you'd expect maximum sensitivity at 90 degrees to the wire, but much less defined lobes. I've actually got something similar in my garden for HF, a kind of dipole, but one element sort of dog-legs around, so it's hardly visible. It doesn't perform as well as the dipole pulled out tight in a line, but with a bit of tuning, is good enough.
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Re: counterpoise antenna

Postby m0lsx » Fri Dec 04, 2015 9:55 am

An earth & a counterpoise are different things, although it is not uncommon to feed an endfed antenna used for transmitting or receiving against an earth stake which is attached to the earth side (the shield) of the coax, but that is largely about electrical isolation & floor noise levels on receive.
Any base fed antenna without a counterpoise is in reality only half an antenna. Every antenna is about balance & half the antenna is the radiator & half the antenna is the counterpoise & if that counterpoise does not exist in a transmitters antenna, then the coax is used as part of the antenna system & that is why some fools think an end fed antenna needs a choke at it's base, it does not, it needs a counterpoise or it needs a choke at the correct point in the coax feed & it may need both.
A quarter wave endfed dipole performs reasonably well with no counterpoise, but 1/2 waves & 5/8th waves etc are not so good & that is one of the reasons that the A99 CB antenna is shunt fed, with a quarter wave length below the feed point of a half wave radiator.
A car provides some metal, but does not make a good counterpoise, hence why radio amateurs add earth straps & tend not to use mag mounts on HF.
If you are interested it is something called Kirchhoff's Law, which is about current & it relates to transmitters. But it is about both sides of the antenna being equal & in balance & it is why a properly made Off Centre Dipole (A Windom) has a RF choke on the coax feeding the antenna & why that choke is not at the feed point, but a distance from it, the coax is used as part of the balance.
An end fed antenna with no counterpoise works perfectly fine on receive & all that adding a counterpoise with an end fed using a balun does is alters where the antenna performs at it's best.
In the largest part on a HF receive antenna the balun is there to provide electrical isolation between antenna & radio. It lowers the background static noise a little.
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Re: counterpoise antenna

Postby m0lsx » Fri Dec 04, 2015 10:21 am

Just to add, a balun is not a single, or simple, thing.
We call something used to provide a choke (that is to stop currents on the coax) a balun, but it is not, although it is in another way, sometimes.
A balun is something that transforms by a known ratio.. 4:1 & 9:1, being common, but they are not the only options. They transform high to low & low to high. It should provide balance in the two halves of the antenna. They are designed for either Voltage or Current, so not all 4:1 or 9:1 baluns do the same thing.
So if you have a very high impedance, your balun could make it lower, but if your impedance is correct it could make it higher. A balun is a compromise when used on something like a general coverage antenna. But then again, most things in an antenna system are a compromise for most of us.
The above is an idea of what a balun is, but they do go beyond that.
And the above is the reason I always say that a balun is about providing electrical isolation between the coax & the antenna & about lowering noise..It will do that, but the rest is less certain & dependent upon us knowing what we are doing.
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Re: counterpoise antenna

Postby lhuchison » Fri Dec 04, 2015 11:26 pm

Thanks for the replies. It does appear to be an improvement , i'll tinker about with it over the next couple of weeks.
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Re: counterpoise antenna

Postby G4RMT » Sat Dec 05, 2015 12:32 am

You almost certainly have a decent earth in the loft - your water pipe - assuming it's copper not plastic - although it can sometime play havoc with your TV reception, as you set up some circulating currents in the pipes that are insulated in stud work walls etc - your TV might object, but digital TV is much more resilient to analogue interference, so worth trying?
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Re: counterpoise antenna

Postby m0lsx » Sat Dec 05, 2015 1:41 pm

G4RMT wrote:You almost certainly have a decent earth in the loft - your water pipe - assuming it's copper not plastic - although it can sometime play havoc with your TV reception, as you set up some circulating currents in the pipes that are insulated in stud work walls etc - your TV might object, but digital TV is much more resilient to analogue interference, so worth trying?


I believe lhuchison is an SWL, not a amateur. So he should have none of those issues. But two issues he may have...
Is the house Protective Multiple Earthed?? If so the whole house could become live if he uses any earth, especially the house plumbing. If there is a problem in his or any other house or the transformer that is part of that system.
Secondly it can introduce QRM into the system.

If anyone is unfamiliar with the Q code.
QRM = Man made interference, such as from LED lights, fish tank pumps, street lights, boiler systems, switch mode power supplies & the dozens of other sources.

QRN = Natural interference, that is the natural background noise from the solar system.
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Re: counterpoise antenna

Postby lhuchison » Sat Dec 05, 2015 3:38 pm

Why are some people against the use of an ATU for receive? i thought for a 10metre end fed, it would help for some of the bands?
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Re: counterpoise antenna

Postby m0lsx » Sat Dec 05, 2015 5:41 pm

For Short Wave Listening an ATU is a great idea. I have a really nice SWL ATU. It is sold under a number of Different badges one being Global AT-1000, the other Mizuhu KX-3. It has band filtering & adjustable Q, so can also most be used for nulling out close by stronger signals.
Any cheap ATU will do & a SWR bridge helps with SWL, as does band filtering.
For scanner use, an ATU serves no purpose at all. As even moving a few khz let alone a few hundred MHz, means readjusting the ATU..
Oh & just to add a ATU does not tune the antenna, it simply lets the radio believe that the antenna is matched & using a manual ATU. Or Antenna Matching Unit as it should properly be called, can have it's own issues, as normally there is more than one point of low SWR & low SWR does not mean a good match between antenna & radio.

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Re: counterpoise antenna

Postby lhuchison » Sat Dec 05, 2015 8:53 pm

I'm pleased you said that as i have a Second Hand Howes CTU8 Receive Only ATU coming monday and can't wait to connect it up!
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