Do scanner antennas require a counterpoise??

[TheJerk]

Do scanner antennas require a counterpoise since they are receive only? I'm planning on stringing a wire antenna up for a temporary antenna at this place...we plan on moving within a year, and I will set up a permanent commercially-made antenna at that point.

I was planning on using a length of RG-6QS, with the shielding stripped back, for both the feedline and the "antenna". Put this in the attic fastened to the rafters...I have over 1500 feet of RG-6QS, the reason I would like to use it.

 

[prcguy]

If you pull the braid back over the coax for 1/4 wavelength and leave a 1/4 wavelength of center conductor, you will have a fine center fed coaxial dipole for one band.
prcguy

 

[nanZor]

If you don't want to cut up your feedline, you could just attach a quarter-wave wire to the center conductor and see how that goes first, as the entire 1500 feet of your shield is a grounded counterpoise due to rf skin effect on the outside of the shield.

Pulling the braid back over itself is a good way to make a temporary antenna, but the biggest problem is that there isn't really enough spacing in the "sleeve" to prevent coupling itself right back into the outer skin of the feedline braid, so you may as well not do it at all unless you have enough spacing. At the very least if you do this, save the jacket you just cut open, and slide that back over the feedline where you are folding the braid back. If you can, try to taper the end of the folded braid away from the feedline, as this is where the coupling effect is the greatest.

Another option is to try and decouple the rf with a few chokes instead of folding back the braid. Three to four of the RS #273-105 snap-on's placed a quarter wave down the coax should do, at least up to about 300 mhz. A bit expensive, but if you have some already or others that are good up to VHF, perhaps give those a shot. I've made quite a few of these by attaching nothing more than a quarter wave to a chassis connector and sliding the chokes on a quarter wave down the coax.

These are obviously single-banded or odd harmonically-related antennas good for quickie installs.

I like the folded-back braid technique, but only if you can get about an inch of spacing between the braid and the feedline, which is usually pretty inconvenient unless you have the spacer material to do it with.

 

[TheJerk]

I understand all that, I've built a dipole for 10M using coax...

But is the counterpoise needed for a scanner antenna? As in receive only?? I was thinking in the scanner application, the shield was nothing more than a way to prevent errant RF in the feedline between the antenna and scanner??

I'm talking receive only, not transmit.

 

[jack103]

Google is your friend!!!

 

[TheJerk]

And that was helpful how?

 

[prcguy]

All very good, often overlooked points for properly dealing with coaxial dipoles. One final nit pick on folding the braid back is the PVC or other type jacket can be a fine weather proof cover for the coax innards but when the braid is folded back over it itself to make a coaxial dipole the jacket under this braid now becomes a dialectic and most jackets are poor and lossy in the dialectic department.
prcguy

If you don't want to cut up your feedline, you could just attach a quarter-wave wire to the center conductor and see how that goes first, as the entire 1500 feet of your shield is a grounded counterpoise due to rf skin effect on the outside of the shield.

Pulling the braid back over itself is a good way to make a temporary antenna, but the biggest problem is that there isn't really enough spacing in the "sleeve" to prevent coupling itself right back into the outer skin of the feedline braid, so you may as well not do it at all unless you have enough spacing. At the very least if you do this, save the jacket you just cut open, and slide that back over the feedline where you are folding the braid back. If you can, try to taper the end of the folded braid away from the feedline, as this is where the coupling effect is the greatest.

Another option is to try and decouple the rf with a few chokes instead of folding back the braid. Three to four of the RS #273-105 snap-on's placed a quarter wave down the coax should do, at least up to about 300 mhz. A bit expensive, but if you have some already or others that are good up to VHF, perhaps give those a shot. I've made quite a few of these by attaching nothing more than a quarter wave to a chassis connector and sliding the chokes on a quarter wave down the coax.

These are obviously single-banded or odd harmonically-related antennas good for quickie installs.

I like the folded-back braid technique, but only if you can get about an inch of spacing between the braid and the feedline, which is usually pretty inconvenient unless you have the spacer material to do it with.

 

[nanZor]

I'm talking receive only, not transmit.

The operation is just as valid as it is for receive. As prcguy mentions, a braid folded back down over it's own covering is not ideal since the jacket doesn't make for a very good rf insulator (and also not the best conductor either, but that's what we'll use). This is the main problem and why an air-gap is the preferred method, even though you can get a folded-braid dipole resonant with trimming. Thing is, it isn't super efficient, albeit better than just a quarter wave whip.

Because of the very poor rf insulation, there is a lot of coupling taking place even on rx-only to the rest of the braid's common-mode. That's why for rx-only applications, I try to "tune" the common mode of the braid with chokes, so it will act like a real quarter-wave element as far as rf is concerned without having to fold back any braid or expose it to the elements.

So for a general purpose antenna that is even quicker to make than a ground-plane, I just stick a quarter wave piece of wire onto a chassis connector and hope that the common-mode of the braid is good enough.

If I have chokes available, I'll tune the braid by placing snap-on chokes that can go up to VHF at least a quarter wave down. All bets are off at UHF though since the chokes aren't effective above 300mhz.

I'll pass on the folded-braid technique, as I find it unnecessary since I have chokes available. AND I don't have to worry about exposing my transmission line to the elements (aside from a light covering of sealant at the connector. Still, I consider these temporary or attic at best and even wrap the chokes with tape.

I hope I'm not misreading your question - it's pointed to the folded-braid vertical dipole for rx-only, but maybe I'm wrong.

 

[TheJerk]

OK, then for a scanner (RX only), a counterpoise is required.

This is going to be a temporary setup...maybe permanent for the house, but I plan to move in the next year or so. When I get to the new place, I will install an actual tower, and install a commercially-available scanner antenna.

I'm thinking I may still use the coax, use three pieces...one for the feedline, one for the antenna, and one for the counterpoise. I have seen dipoles made from coax, where the outer braid is used as the "hot" (I think the common name is Double Bazooka?). Just trying to keep this on the cheap, and I have a lot of RG-6QS sitting around.

I think I have around 20 feet of room in the attic, probably make a folded dipole with each leg in the shape of a "U", the entire dipole in the shape of a flat "S". Maybe make it a "Z" shape...10-feet to the wall, and follow a rafter as far as I can?

 

[TheJerk]

If you don't want to cut up your feedline, you could just attach a quarter-wave wire to the center conductor and see how that goes first, as the entire 1500 feet of your shield is a grounded counterpoise due to rf skin effect on the outside of the shield.

I just noticed this...I don't intend to use the entire 1500 feet, it was merely a reference to indicate I had a lot of this particular coax at hand.

I intend to only use the length that is needed to reach from the radio to the antenna (about 20 feet) and then whatever was needed to build the "antenna".

 

[prcguy]

Keep it simple and make a ground plane with 3 or 4 radials. With all elements around 18" you will have a VHF antenna that will work ok at UHF also.
prcguy

A lot of RG-6 has a token braid that does not
solder, almost like its aluminum.

OK, then for a scanner (RX only), a counterpoise is required.

This is going to be a temporary setup...maybe permanent for the house, but I plan to move in the next year or so. When I get to the new place, I will install an actual tower, and install a commercially-available scanner antenna.

I'm thinking I may still use the coax, use three pieces...one for the feedline, one for the antenna, and one for the counterpoise. I have seen dipoles made from coax, where the outer braid is used as the "hot" (I think the common name is Double Bazooka?). Just trying to keep this on the cheap, and I have a lot of RG-6QS sitting around.

I think I have around 20 feet of room in the attic, probably make a folded dipole with each leg in the shape of a "U", the entire dipole in the shape of a flat "S". Maybe make it a "Z" shape...10-feet to the wall, and follow a rafter as far as I can?

 

[TheJerk]

So the extra length isn't needed?

I can build something like this pretty easily, and then let it hang from the rafters (assuming four radials)...

 

[TheJerk]

A lot of RG-6 has a token braid that does not solder, almost like its aluminum.

I tested mine, you were correct in that it cannot be soldered...I've soldered aluminum braid before, but there was no soldering this. I use regular RG-8 with copper braid,and never dealt with TV coax before.

I did throw a crude wire antenna up, but it isn't a dipole (yet). This new GRE400 doesn't seem as sensitive as my GRE300 with a "super gainer" antenna (not comparing to my home-brew). My wire antenna seems to work well, but I'm planning to buy an ST-2 to place in the attic, again; this is only a temporary thing. I had really good luck doing that before.

I can't wait to move so I can set up a permanent tower and antennas.

 

[TheJerk]

I turned my wire antenna into an oversized dipole tonight...made little difference, if any, in the receive. There was nothing lost by doing this, and I wanted to see if it actually made a difference.

Not that it matters, should have the ST2 this week.