Antenna mounting can be a tough subject. We want multiple radios on the air monitoring different frequencies and bands, yet we are limited by the amount of space available to place antennas. A common question I receive is how close can I mount this antenna to my other VHF, UHF, or HF/CB antenna. With most questions of this nature, there is an easy answer and a not so easy answer.
The easy answer is that you should maintain a minimum mounting distance of at least 1/4 wavelength apart for the lowest frequency antenna within the same horizontal plane. For VHF antennas, that translates to roughly 19 inches. This is the rule we use when installing omni antennas on a the trunk of a vehicle. This guideline seems to work fine and with all the multiple radios I’ve used in my cars, and other than experiencing signal desense while a radio was transmitting; I’ve never had issue of one radio destroying the other.
What about base station operation? Will this rule work for antennas mounted on a mast or tower? It should, but I like to maintain the maximum distance possible. Just because two antennas can co-exist on the same plane, doesn’t mean that they should. Separating two antennas by three feet will cause a 30db drop in signal between the two antennas. So transmitting 50 watts on one transmitter could cause as much as 50 milliwatts of energy to travel down the feedline of the second antenna. For every foot of spacing, you’ll receive another 10db drop in power. The further apart you can separate the antennas, the lower the interaction between the two antennas will be.
Vertical separation can allow two antennas on the same frequency range to co-exist
Separating antennas horizontally can take up quite a bit of real estate. Getting the antennas out of their transmit apertures will greatly limit the cross-talk or desense between the two transceivers. Omni-directional antennas like the 2 Meter J-Pole or 2 Meter Slim Jim antennas have a donut shaped radiation pattern. If you can move the second antenna outside of the donut, you can limit the amount of desense. The easiest way to do this is two mount the two antennas vertically, one higher than the other. For each foot of vertical separation you can realize 10 feet of horizontal separation.
MFJ-801 Field Strength Meter
Finally, antennas on two different bands are less likely to interact with each other as compared to antennas on the same band. Your 2 meter antenna isn’t resonant on HF frequencies and your HF antenna isn’t resonant on the 2 meter band, so their proximity isn’t much of an issue than two VHF antennas next to each other.
The formulas and distances of course are theoretical and other outside factors like buildings, structures, other antennas, towers could affect how your antennas receive and transmit. The best way to test for crosstalk and desense is to use a Field Strength Meter. this device will give you a real world number of the electromagnetic energy emanating from an antenna at a certain distance. Field strength meters can run a large price range, depending on their sensitivity and calibration. But an inexpensive meter like the MFJ-801 is sufficient for situations like this.
Dear Sir,
I am in need of an improved antenna for FM radio reception. Would one of your models work for this purpose? Excuse my ignorance, as I am new to this science. Thanks, Stu Cummings
Hello Sir… I have some questions..
I want to mount two FM tranmitter antennas in the same tower… But whit a relative high power, about 1KW. Is te same analysis ???
Leonardo,
The frequency of your radio system can affect the separation distance of the two antennas. Separation will be even more important as you increase your transmitter power. Here are the charts and data that I used to create this article. You can use this info to find an optimal separation for your antennas.
http://www.repeater-builder.com/antenna/separation.html
I hope this helps you out.
Pleas tell me if 2 VHF radios can be used on one antenna and if so how would I do it.
Thanks for any help.
Gary,
The easiest way for two radios to share the same antenna would be to use an antenna switch. A more elaborate solution would be to use some type of diplexer. I’ve got a blog post that talks more about the subject:
http://www.jpole-antenna.com/2012/07/12/can-i-share-my-antenna-with-2-radios/
Sir I am proposing to use a 2mtr and 70cms slim Jim on one mast re spacing which antenna do I put at the to is 70cms at top what do you advise regards john
John,
If you are mounting the two antennas vertically, I’d just make sure that they are clear of each other by at least a foot. You’ll want to mount the UHF antenna above the VHF antenna to give it a little better range. If they will be mounted horizontally on the same side-arm, then make sure they are at least 1/4 wave apart of the lowest frequency. That would figure to be about 24 inches (although more is always better).
Michael
Hello. At my home to rx TV and radio I have a Yagi UHF antenna and a FM circular antenna beneath. They are separated 50 cm apart, UHF on top. These 50 cm are counted from the middle of the Yagi antenna and not from the bottom of his lower reflector. Are 50 cm enough? And am I making a mistake in counting them not from the bottom of the Yagi but from his central axis? Thanks
Here is a picture from my scenario: http://imgur.com/ZRpBtXX
Thank you so much!
50 cm is fine for these antennas. Since both are being used for receive only, you aren’t going to run into problems with transmit interference. You should be fine in this situation.
Michael
Hello Michael and thanks. The 50 cm are counted from the middle of the Yagi antenna and not from the bottom of his lower reflector. So the distance between the FM antenna and the Yagi’s bottom will be less than 50cm. Probably not even 10. Am I making a mistake in doing this?
The distances in your case aren’t critical because you aren’t transmitting on the antennas. It becomes an issue when one antenna is a receiver and the other is transmitting. But to answer your question: you should calculate the distance from the ends of the antennas, not the center of them.
A lot of good info. But how close can I mount two 2m jpoles antennas that are horizontal separation? I want to use 2 jpoles to run a 65-watt FM and a 50-watt DMR at the same time on 2m.
Thank you for any help.
73s w5dzg
If you are planning to run two high power radios in the same band at the same time, I’d recommend an attenuation of of 40db or more. That means that your will need to separate the antennas 75 feet horizontally, or 15 feet vertically. You could put the antennas closer, but you’ll find that you will run into overloaded front ends on one radio while the other one transmits.
Here’s a chart that will help you decide how far apart to space your antennas:
http://www.repeater-builder.com/antenna/separation.html
Michael