Antenna/Neighboring Feedline Mutual Inductance

[O-B-1]

I have a Comet CX-333 triband (144/222/440MHz) antenna mounted at top of a 40 ft fiberglass mast. The LMR-400 feedline runs down the mast on side opposite antenna. Directly below the CX-333 antenna is mounted a Comet CA-712efc GMRS (462-467MHz) antenna. The feedline for the GMRS antenna is routed to the opposite side of the mast and next to the amateur band feedline from the antenna above.
If I have both radios turned on and transmit on the GMRS radio, it breaks squelch on the amateur radio when it is receiving in the 440MHz band.
The lower (GMRS) antenna is only 3" away from the feedline from the upper antenna. I think it is "coupling" with the feedline and creating common mode currents on the outside of the shield.
I am thinking some snap on ferrite beads at the upper antenna feedpoint, and just below the lower antenna feedpoint on the upper antenna feedline may attenuate this problem, somewhat.
Any thoughts, advice or suggestions on this issue?

Thank you in advance, 73 DE KJ7WUZ
Dave

 

[mmckenna]

That may be part of the problem, but I expect you have your antennas too closely spaced and the transmitter of one is overloading the receiver on the other.

You need more separation between the antennas, and you need to stand the lower antenna off from the mast.

 

[O-B-1]

I researched it and found documents stating a separation of 3m center-center at 2m. But they were illustrating collinear folded dipole array above another. The antennas I am using are both high gain collinear verticals. That's why I thought it was the mutual Inductance from lower antenna to the shield of the upper antenna feedline. The references I found also directed having antennas directly one above the other. Putting more distance between lower antenna and coax would probably go a long way in reducing the issue, agreed. So your idea of the lower antenna being on a standoff seems sound.

 

[O-B-1]

That may be part of the problem, but I expect you have your antennas too closely spaced and the transmitter of one is overloading the receiver on the other.

You need more separation between the antennas, and you need to stand the lower antenna off from the mast.

Any idea how much further down the mast the lower antenna should go?
Thank you, Sir

 

[W5lz]

I don't think the inteference is cause by the feed lines or antennas. More likely the two (or more) radios are the culprits, lots of RF when transmitting.

 

[mmckenna]

Any idea how much further down the mast the lower antenna should go?
Thank you, Sir

Too many variables involved. Your best bet is to experiment. Try lots of horizontal separation, like putting the antennas on opposite sides of the house.

 

[K5MPH]

I think that the Ham radio front end is being overloaded and not coupling of the two feed lines........

 

[O-B-1]

Too many variables involved. Your best bet is to experiment. Try lots of horizontal separation, like putting the antennas on opposite sides of the house.

Sounds like being a boat owner, <laughing> Guess I had better start saving up for a reel of LMR-400

 

[mmckenna]

Sounds like being a boat owner, <laughing> Guess I had better start saving up for a reel of LMR-400

Yeah, pretty much.
There's a ton of variables involved, and figuring them all in would get you close, but then there's all the variables you don't know.

Ham radios with wide open front ends to provide "wide band coverage" don't have the filtering to block/reduce the out of band signals.
Also, if I'm understanding the mounting correctly, you'd still need to stand off the GMRS antenna a foot or two from the mast to keep the other coax from interfering with the pattern of the antenna.

Kill two birds with one stone and just put them on different supports.

 

[WA0CBW]

Also it takes more than double the horizontal separation to equal the same same amount of vertical isolation.
BB

 

[W5lz]

This is an opinion so take it for what it's worth;

Coaxial feed lines generally get a bad 'rap' when it's not really the coax's fault. If certain types/sizes of coax are really that bad, why are they still around? The different types of coax -do- have 'plus/minus's, but going to the next 'better' coax seldom makes any discernible difference in performance. The two 'biggies' are impedance and loss. Of those two (assuming impedance is close) the mount of loss per 100 feet isn't that big'a problem... but keep it in consideration. Four or five hundred feet of RG-58 just isn't that good of an idea, right? Neither is hardline for that matter.
There's also a 'third' consideration, at least for me. That's -cost-! I'm not ever going to be running hardline to an antenna, I can't afford it. If you can... knock yourself out with it.

...oh, and anything more than a 1/4 mile separation is wasted.

 

[jeepsandradios]

What do you mean cable loss doesn't mean anything ? RG58 at 450mhz is 9.87db of loss per 100' LMR400 is 2.7db and 100' of LDF4-50 is 1.4db of loss. To me even 3db of loss is alot better than 10. Maybe I'm missing what your saying but doesn't seem right to me.

 

[prcguy]

The coupling between two close spaced runs of LMR400 is very small at VHF/UHF and probably in excess of 150dB. The coupling into one LMR400 feedline with an antenna 3in away is whatever the power level is at the antenna plus the gain of the antenna minus the shielding properties of the coax, which is greater than 90dB. The coupling between two antennas spaced a few feet apart vertically is not going to be much, maybe 20 to 30dB max and that would be your worst offender.

There probably is some common mode currents riding on the coax induced by the antenna blasting right into it but the close proximity of the antennas will still be the big problem. You would have to get the antennas maybe 20-30ft apart vertically and test for coupling and then you could throw on some ferrite like 43 mix snap on and at least three in series to see if there is any slight improvement.

Unless your using some high end Motorola or other bullet proof radios you will probably have to put a band pass filter on one or both radios to keep them from interfering with each other. I would also get the lower GMRS antenna spaced out from the mast and coax at least 2ft or 1 wavelength as the coax 3" away is making the omni vertical into a 2 element directional beam antenna.

 

[W5lz]

What do you mean cable loss doesn't mean anything ? RG58 at 450mhz is 9.87db of loss per 100' LMR400 is 2.7db and 100' of LDF4-50 is 1.4db of loss. To me even 3db of loss is alot better than 10. Maybe I'm missing what your saying but doesn't seem right to me.

And would you use RG-58 at 100 feet rather than LMR-400 at that frequency? Is the amount of loss the only criteria you're going by?

 

[jeepsandradios]

@W5lz I guess I'm confused on the point your trying to make. No way I would use RG58 on UHF at that distance. To be honest if I'm going 100' I'm going to use 7/8" LDF if possible.

 

[W5lz]

That last question is what I'd like to know, is the amount of loss the only thing you're looking for?

 

[merlin]

And no one mentions grounding of the antennas. the bases should be grounded as best you can.
Vertical separation, as long as they don't overlap should work well being different bands.

 

[buddrousa]

Just look at CELL TOWERS 60 to 80 feet of VERTICAL SEPERATION between different carriers on the same tower.
Our big tower here has 60 foot of vertical separation between same band antennas and feedlines are on the same face 3 inches apart grounded at the top at the bottem and at the plate where it enters the shack.

 

[O-B-1]

Another factor I forgot to consider is the TYPE of antennas I am using, versus what the documents I was studying for this. They are discussing the separation distances between radiation centers of 1/2-wave dipoles, I'll wager. The natennas I am using are collinear (stacked 1/4 and/or 1/2-wave array in the tube). SO perhaps, wjat is needed is a 3m sparation between the ENDS of the antennas, and NOT the CENTERS. I overlooked that. The references I found actually stated that each foot separation in the vertical for VHF/UHF is worth 10 feet horizontal.

Just look at CELL TOWERS 60 to 80 feet of VERTICAL SEPERATION between different carriers on the same tower.
Our big tower here has 60 foot of vertical separation between same band antennas and feedlines are on the same face 3 inches apart grounded at the top at the bottem and at the plate where it enters the shack.

Thank you for that information, Sir! QSL that...

 

[O-B-1]

And no one mentions grounding of the antennas. the bases should be grounded as best you can.
Vertical separation, as long as they don't overlap should work well being different bands.

True enough... But the GMRS is only 12.550MHz fruther up than the 70cm. I will ensure I have them well grounded. I am running a ground wire from mount to mount, then down the mast to the SPGP.