Two Antennas On The Same Mobile Roof

[22racer]

I have my Wilson 5000 antenna in the middle of my ext cab pickup roof and want to mount another antenna tuned for 160 MHz (18 inch?) behind it. How far apart do these antennas need to be, or does it really matter that much? The 160 MHz will be for recieve only.

 

[northern35s]

I have my Wilson 5000 antenna in the middle of my ext cab pickup roof and want to mount another antenna tuned for 160 MHz (18 inch?) behind it. How far apart do these antennas need to be, or does it really matter that much? The 160 MHz will be for recieve only.

An insignificant amount of interaction, I thought I'd model it just to see how much interaction, hardly visible is the conclusion. Whenever we put any metal object in the near field of an antenna there is the possibility of mutual inductance, especially if the metal is in the same polarisation and makes up a significant portion of a wavelength, the little 18" antenna doesn't however make up a significant portion of a wavelength so it's influence is almost invisible at 27MHz

 

[W5LZ]

I agree that there shouldn't be much interaction between two antennas on the bands you would be using, but there will be some interaction. Placing those two antennas a "reasonable" distance apart is a good idea. What's 'reasonable'? If possible, something like the length of the shorter one from the taller one, what's that, about 15 - 18 inches?
Some of the possible 'interactions' between the two depend on what the second antenna is connected to. If it uses FM mode, there can be some desensing of that second receiver. The 6th harmonic of 27.2 Mhz is 165 Mhz, which can also be a possible 'problem' at times. And any two antennas near each other are going to affect both's resonant frequency to some extent. Nothing that can't be compensated for, and it shouldn't be much, but it still happens. I don't think it would be a 'biggy' at all though. Haven't got 18"? How about close to that?
- 'Doc

 

[Beetle]

Had a few "interesting" hiccups when doing 2M installations with some friends. They had cars in which the AM/FM "entertainment" radio antenna would normally be retracted and would pop up only when the entertainment radio was turned on.

Significant percentage of wavelength suddenly appearing just a foot away from the 2M antenna?! You've never seen an analyzer behave quite like that! Took us a while to figure out what was causing the trouble, but it's valuable experience for when the next time it happens.

 

[Robb]

Use a 10/11m antenna just 4ft away from a 2m/70cm antenna on my vehicle all the time - and never had any issue with either radio.

Of course, never ran both radios on at the same time . . .

 

[northern35s]

I agree that there shouldn't be much interaction between two antennas on the bands you would be using, but there will be some interaction.
---snip---
And any two antennas near each other are going to affect both's resonant frequency to some extent. Nothing that can't be compensated for, and it shouldn't be much, but it still happens. I don't think it would be a 'biggy' at all though. Haven't got 18"? How about close to that?
- 'Doc

It does affect resonance, my simple model from this afternoon, 1/4WL CB antenna over a decent groundplane:

R30.93 j2.177 with short receive antenna installed
R30.96 j2.173 without short receive antenna

Like I said, such an insignificant amount it would be impossible to see any difference in use.

Now if Henry or Marconi would let me have a copy of the Eznec car, it may move me to buy Eznec

 

[Marconi]

It does affect resonance, my simple model from this afternoon, 1/4WL CB antenna over a decent groundplane:

R30.93 j2.177 with short receive antenna installed
R30.96 j2.173 without short receive antenna

Like I said, such an insignificant amount it would be impossible to see any difference in use.

Now if Henry or Marconi would let me have a copy of the Eznec car, it may move me to buy Eznec

35, email or PM me and I'll email you Henry's Bronco model or my Pickup model. My P/U has a 6" rear bumper

 

[The Jerk]

Coming from someone who did a similar install, it will likely make the CB directional to the rear.