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CB Radio Forum - Discussions regarding Citizens Band Radio (CB)

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  #21 (permalink)  
Old 02-08-2018, 6:25 AM
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Originally Posted by Token View Post
I don't believe anyone was talking about adding loading coils or inductors to a half wave dipole. So far it sounds like we are talking about an unloaded dipole.

T!
You can't fit a 17-foot vertical dipole in any residential attic space I've ever seen, and keep it vertical as required for CB. Laying it horizontal or doing an inverted V will degrade performance and make the antenna directional.

A 1/4-wave vertical ground plane with a loading coil wound to just fit in the available space will perform better than any dipole in this situation.
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Old 02-08-2018, 9:11 PM
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Originally Posted by prcguy View Post
You mention your dipole is 17' 2" on each leg? As in 34' 8" total? A 1/2 wavelength dipole would be 1/2 wavelength total length and fed in the center for about 8' 6" or so each leg.
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I got that wrong, it's overall length is 17' 2.4". Each leg is 8' 7.2"
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Old 02-08-2018, 9:16 PM
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Originally Posted by Token View Post
prcguy hit the nail on the head here. Your TOTAL half wave dipole (or inverted V) length should be about 17 feet 3 inches as a starting point. Try starting with your antenna about 8 feet 8 inches on each side, or about 17.2 feet total. Then check the SWR and don't be afraid to tune it. Starting at 8 feet 8 inches should make the antenna a little long, but far easier to trim to tune then to add to tune.

T!
I got that wrong, it is 8' 7.2" on each side.
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Old 02-09-2018, 10:34 AM
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Originally Posted by jonwienke View Post
You can't fit a 17-foot vertical dipole in any residential attic space I've ever seen, and keep it vertical as required for CB. Laying it horizontal or doing an inverted V will degrade performance and make the antenna directional.
Vertical polarization is not a requirement for CB. It is absolutely true that almost universally mobiles on CB are vertical. And so for local communications, particularly if you are going to work local mobiles, vertical is desired. So you can validly say vertical is the most common polarization on CB and is generally the most desirable for local work. But horizontal can be, and often is, used on CB.

Many DX stations use horizontal. And the polarity of a DX signal is often chaotic, meaning even if a DX signal started out vertical there are many times you can work it best if you are horizontal (also the other way around, if it started horizontal it might be best receiver vertical). If you are talking locally base to base and both are horizontal that will work just as well as vertical.

But you are missing the point of the inverted V antenna. It is NOT necessarily horizontally polarized. And it is rather less directional than a horizontal dipole.

When you look absolutely perpendicular to the axis of the element, I mean directly broadside, the inverted V is indeed horizontally polarized. But off each end it is vertically polarized. And as you move from these two locations (actually 4 locations) the polarity starts to shift towards the other plane. This means that for the majority of the way around it is actually slant polarized. This is both a benefit and a burden.

45 degree slant polarization costs you about 3 dB of signal when talking about either a horizontal or vertical signal that is direct path. This means it does both polarities with a slight degrade. But 3 dB is about half an S unit, not a huge hit.

So with an inverted V you get an antenna that, for local communications, is not quite as good as either a pure vertical or horizontal antenna is when dealing with signals of those polarities. But it will often do signals of both polarizations quite acceptably. And for chaotically polarized DX signals it (the inverted V) is often just as good as either pure polarization antennas are.

Another advantage of an inverted V is it is often more simple to get it higher than other types of antennas, since its main support can be a rather simple hanger at the apex.

Quote:
Originally Posted by jonwienke View Post
A 1/4-wave vertical ground plane with a loading coil wound to just fit in the available space will perform better than any dipole in this situation.
But now you have loaded the quarter wave, and didn't you say something about loaded antennas being less efficient and reducing bandwidth? And it will NOT work better than a dipole if the user wants horizontal polarization or can use directionality to his benefit.

The fact is an antenna mounted in the attic is going to be a compromise, no matter what kind of antenna it is. And the inverted V itself is a compromise antenna, not really as good as either a pure vertical or a pure horizontal antenna, but sometimes you can use this to your advantage.

When I lived in a town house I had this same issue, no outside antennas allowed and an attic space to work with. I tried dipoles, I tried both 5/8 and quarter wave ground planes (loaded, as I had 7' 6" to work with), and I tried inverted V's. I ended up finding that no one antenna worked best, and my best configuration for general purpose CB use was 2 inverted V's, mounted 90 degrees from each other. This gave me both horizontal and vertical polarizations pointed in directions I could select. Later I fed the two of them from a single feed point, and no longer had to switch them. In my case I think one of the reasons the inverted Vs worked better than the loaded GP antennas I tried was that I could get the antennas a bit further from the floor of the attic and I had more latitude on where they went in the attic. The floor of the attic was the ceilings of the rooms below, with wiring and ductwork present. So I was able to get the antennas a few feet further from noise sources and metal objects that impacted the performance of the antennas.

Inverted V's are simple, it will not cost the OP much to try it out. And of course he should try the others mentioned also, including a loaded ground plane.

T!
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Old 02-09-2018, 2:49 PM
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Between the directionality and the polarization shift, a minimally-loaded 1/4-wave ground plane is likely to perform better than the inverted-V dipole, unless you're shooting skip. Installing a 1/4-wave ground plane to fit into a 7- or 8-foot attic space would require minimal loading, possibly none.
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