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Stacking verticals on single mast.

Jan 31st 2022, 23:04


Joined: Mar 31st 2020, 15:11
Total Topics: 0
Total Posts: 0
Landed my Technician ticket 9 months ago.

Comet CX-333 (VUF/UHF) antenna at top of mast.

Comet CA-712EFC (GMRS) antenna mounted directly below and tip 12 feet below the ground plane of the CX-333.

I have researched the topic and found that UHF separation of antennas should be minimum of 10 feet, radiation center to radiation center. BUT... the references are for dipole antennas.

The antennas I am using are high gain (6-9dBi) collinear, hence I was thinking of putting at least 10 feet between ENDS of the antennas.

I am concerned that the lower antenna will induce common mode current onto the outer surface of the shield of the feedline to the upper antenna due to close proximity and mutual inductance. Is this a possibility?

The same documents show horizontal separation would need to be around 10x greater, which on my small lot is not feasible (not to mention the expense and loss to get the feedlines the length needed.

I have not yet found a high-Q low-pass filter, and probably won't be able to since the UHF radio reaches 450MHz and the GMRS starts at 462.550MHz.

Can this be mitigated by using ferrite bead chokes of the proper mix at the upper antenna and perhaps just below the ground plane of the lower antenna? Or am I trying to locate the "Holy Grail" with this?
Feb 1st 2022, 08:22


Super Moderator

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
This free program may be used to determine the interaction between the two antennas. It is a lot of work to do that, so many hams would just try it out and see if it works well enough for them.

Vertical stacking results in the mast blocking the lower antenna in one direction. One way around this is to install several antennas around the mast.

Surplus and new 70 cm bandpass duplexers could be used to separate GMRS and ham signals. Normally they are just used as ham or GMRS repeaters filters.

Zak W1VT
ARRL Senior Lab Engineer
Feb 1st 2022, 10:36


Joined: Mar 31st 2020, 15:11
Total Topics: 0
Total Posts: 0
Thank you, W1VT and noted! I will look into the diplexers and check used market for a source. I will post my results after trying the solutions provided.

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