Closeness of Antennas

[ofd8001]

I have a Radio Shack discone antenna on my roof mast. Just beside it, there is a Scannertenna. They seem to be working pretty good, just inches apart from each other.

However, I was wondering if that's too close and if I spread them out, which may be a challenge, they would work better.

Thanks

 

[DaveH]

Yes, the antennas are too close and will interact with each other. The impedence of one will be altered by the other although this may not be the most important factor for wideband antennas that are not particularly well impdedence-matched at every frequency anyway.

One antenna will affect the "radiation pattern" of the other which can create nulls which cause loss of signal in some directions. Conversely it might help in some cases but it is not very predicatable. Precise spacing of identical antennas with control of the phasing between the two (through the coax) can create some interesting "patterns" such as cardiods (with a null), figure-8 etc., but not with two very different antennas.

If possible, separate them by several wavelengths at the lowest frequency. At VHF high-band, a wavelength is roughly 6 feet. It's not always feasible, just do what you can.

Dave

 

[bluesman905]

Yes, the antennas are too close and will interact with each other.

Dave, is this also an issue with mag mounts on a vehicle?

 

[jim202]

As Dave has pointed out, spacing of antennas can effect the pattern of their operation. On the receive side, you probably can't notice very much of this. When used for transmitting, the closeness will cause problems with the SWR and the strong radiated energy getting into other radios. This could cause damage to the receiver front ends.

When roof mounted mag mounts are used the same issue is there. Only problem is that you don't have the space to get much separation. The bottom line is on receive, I wouldn't make much out of it, but if used for transmitting, your going to need to space them out.

Jim

Dave, is this also an issue with mag mounts on a vehicle?

 

[Al42]

Spacing vertically polarized antennas (such as scanning antennas) vertically (one on top of the other) gives an inherent additional 10db or so of isolation, so spacing can be closer. Separating 2 scanning antennas that cover VHF-hi by 6 feet is adequate, while spacing them 12 feet side-by-side really isn't. Which is why, on a commercial tower, you'll see antennas mounted at different heights, but rarely a few antennas on the same side of the tower at the same height.

 

[flyingwolf]

Along these lines I saw a Salvation Army truch come through our parking lot today with 2 antenna's on it.
Judging from the heights I would say 800Mhz, 150 range, 400 range, and a Giant 102 inch CB whip.

All of these were places within 2 feet of eachother.
I dont mean two feet apart.
I mean from the first to the last there was less then two feet.

Huge carrier van and all that space on top and they place them all in a 2 foot section.

 

[wesct]

Spacing vertically polarized antennas (such as scanning antennas) vertically (one on top of the other) gives an inherent additional 10db or so of isolation, so spacing can be closer. Separating 2 scanning antennas that cover VHF-hi by 6 feet is adequate, while spacing them 12 feet side-by-side really isn't. Which is why, on a commercial tower, you'll see antennas mounted at different heights, but rarely a few antennas on the same side of the tower at the same height.

when the local repeater club had a noise problem with the antenna on the tower, they decide to use 2 antennas. receive on the top and right below the transmit antenna (upside down of course).
the noise went away and all is fine now.

when i put up antennas at my parents house, i seperated the antennas a quarterwave apart at the lowest frequency. and it worked fine.

wesct

 

[kb2vxa]

Hi bluesman and readers,

"Dave, is this also an issue with mag mounts on a vehicle?"

Here's a more specific answer. How the antennas are mounted doesn't matter one bit. If the antennas are tuned for different bands (scanner antennas aren't) there will be little pattern distortion, they are invisible to each other. If they are tuned to the same band the non active ones act as parasitic elements making one big directional antenna. That's how directional arrays work whether the elements are active or parasitic. Oh, parasitic means that the inactive ones are "energized" by the active one.

Here's an interesting note on how AM broadcast stations eliminate pattern distortion caused by large nearby metal objects such as other towers and water tanks etc. They are "detuned" by running wires up the tower on insulated spacers and connecting them to the tower at the top. They are commoned at the base and connected to a well grounded antenna tuner of the sort used to tune the broadcast towers themselves. The tuner is adjusted in such a way so the tower becomes invisible to radio waves at the frequency it's tuned to.

 

[ofd8001]

Thanks for the ifno

Thanks to all of you who replied. My fund of knowledge has been improved!

 

[DaveH]

Just a comment about vertical spacing...yes you will get much better isolation than the same amount of horizontal separation. However, the extra height ususally means a mast or tower, which some situations don't allow or is otherwise undesirable i.e. attracts attention or lightening. Also, antennas mounted off the mast/tower will be affected by the proximity to metal. Commercial antennas do this all the time but factor it into the antenna's behaviour; whether that happens with scanner antennas is another matter.

And of course, vertical spacing is pretty well out of the question for mobile installations. I use an antenna on the trunk plus a mag mount on the roof which can be removed. Sometimes a third antenna is placed on the roof for trips etc.

Dave