DaveInDenver
Rising Sun Ham Guru
FYI, the SBB-1 is a 1/4λ on 2m so it may not tune well on your bumper. It'll need a ground plane to work. The SBB-5 is 1/2λ on 2m so should be ground plane indifferent. Be interesting to see how it works out.
What HAM antenna? Thinking front bumper this time.
- Thread starter Corbet
- Start date
I figured the SBB-1 will end up on the magnetic mount and then can be stuck down on the center of my hood if need be. 
Parts arrived yesterday. So in a final thought @DaveInDenver is there any issue with mounting both the CB and HAM antennas within maybe 6" of each other? Thinking about installing a couple mounting tabs on my swing out behind the spare tire. This would put the base of the antenna right at the hub of a 37 protecting about half the antenna length between the spare and body while the tip would be about the same height of my RTT preventing it getting snagged by trail trees. Both coax would have to run basically paired to the radios.
DaveInDenver
Rising Sun Ham Guru
@Corbet, as it happens the question of antenna proximity has been raised. Over on ExPo I posted some information.
expeditionportal.com
Instead of reposting the whole thing I'll just answer your specific question with an additional graphic.
I would caution putting them 6 inches apart*.
*See caveat at end...
The expanded explanation. When I was doing that experimenting I measured 54 dB of isolation between a Larsen NMO27 and my monoband Flexiwhip when spaced roughly 20 inches apart. That means at 50 watts (+47 dBm) VHF the out of band power at the CB would be -7dBm, which the filtering should take care of and probably not be a problem.
So then at 6 inches, well I'm not how much higher the RF coupling will go but my suspicion is it would still be tolerable.
Why I would urge caution is isolation goes to nil if the whips physically touch. There's not really any safe power level in that case.
If you're using those fiberglass rigid masts with the spring bases then it might be alright, though. I wouldn't put two stainless whips or a whip with a wire winding around (e.g. Firestick) that close. And that's saying something because I *did* put two VHF/UHF antennas within 20 inches of each other and for that I should be kicked in the shins.
GMRS, CB, and AM/FM antennas in close proximity?
I prefer not to do roof-mount antennas for the moment (don't want to drill holes, and don't like wires running across the roof), and don't like antennas in front of the cab (personal preference). So I'm considering fabbing up an antenna platform that would sit above my spare tire in the rear...
expeditionportal.com
Instead of reposting the whole thing I'll just answer your specific question with an additional graphic.
I would caution putting them 6 inches apart*.
*See caveat at end...
The expanded explanation. When I was doing that experimenting I measured 54 dB of isolation between a Larsen NMO27 and my monoband Flexiwhip when spaced roughly 20 inches apart. That means at 50 watts (+47 dBm) VHF the out of band power at the CB would be -7dBm, which the filtering should take care of and probably not be a problem.
So then at 6 inches, well I'm not how much higher the RF coupling will go but my suspicion is it would still be tolerable.
Why I would urge caution is isolation goes to nil if the whips physically touch. There's not really any safe power level in that case.
If you're using those fiberglass rigid masts with the spring bases then it might be alright, though. I wouldn't put two stainless whips or a whip with a wire winding around (e.g. Firestick) that close. And that's saying something because I *did* put two VHF/UHF antennas within 20 inches of each other and for that I should be kicked in the shins.
That’s what I figured. I’ll mount my HAM up front and move my CB to the back when I have time. It’s currently right next to my FM factory antenna. So 3’ from where the HAM will end up.
RDub
Locked
Hey, Dave. I just read through your series of posts on ExPo. Great information, thanks. You stated "A dual band 2m/70cm ham radio wouldn't tolerate a GMRS antenna in close proximity." That makes sense to me. You also mentioned turning a radio off doesn't really protect it from potential damage when an offending signal is strong, such as having two antennas very close together. So my question is what about HT's? Many of us have a GMRS HT and 2m/70cm HAM HT along with us. Are they more or less protected from each other due to low power, or is there a potential for damage when, say, the HAM is turned off and one is transmitting on the GMRS with the radios fairly close to each other?
DaveInDenver
Rising Sun Ham Guru
@RDub, please correct me if my assumptions are wrong. You are asking (I think) if you need to worry about damaging one HT when you transmit on the other in a "normal" way. You're not talking about an HT that you've connected to an external antenna, right?
If that's the case then, yeah, you should think about it but it's not necessary to worry much.
There's a few gotchas in all of this - the amount of power, the antenna gain, bands you're using, proximity and orientation. The worst case is lots of power into antennas close and parallel to each other on the same band, which is what I'm talking about with 50 watt mobile radio(s) into this situation. This is a definition of "Do as I say, not as I do." if ever there was one.
With HTs the power is 10 dB lower (5 watts is +37 dBm for calculations) and the antennas usually have around 2 to 3 dB less gain. Then moving them so they aren't so close and parallel makes a major difference in coupling. If you're using a GMRS HT and a VHF/2m monoband HT then you have out of band filtering on each that helps.
Bottom line is you mostly don't need to be concerned.
The one situation I might caution is if you're using a speaker/mic with your radio(s) and have them both sitting next to each other in a cup holder or something. That would be potentially a tough situation to do a lot.
If that's the case then, yeah, you should think about it but it's not necessary to worry much.
There's a few gotchas in all of this - the amount of power, the antenna gain, bands you're using, proximity and orientation. The worst case is lots of power into antennas close and parallel to each other on the same band, which is what I'm talking about with 50 watt mobile radio(s) into this situation. This is a definition of "Do as I say, not as I do." if ever there was one.
With HTs the power is 10 dB lower (5 watts is +37 dBm for calculations) and the antennas usually have around 2 to 3 dB less gain. Then moving them so they aren't so close and parallel makes a major difference in coupling. If you're using a GMRS HT and a VHF/2m monoband HT then you have out of band filtering on each that helps.
Bottom line is you mostly don't need to be concerned.
The one situation I might caution is if you're using a speaker/mic with your radio(s) and have them both sitting next to each other in a cup holder or something. That would be potentially a tough situation to do a lot.
RDub
Locked
Thanks, Dave. Yeah, that's what my long-winded post was actually asking.