Antenna spacing question for APX 7500

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May 19, 2019
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I have an APX 7500 (VHF, UHF R2) I am looking to get installed. Before I do, I have a few questions about antenna spacing on a 2011 Ford Explorer. I will be using two antennas for the different bands on the radio, one is a quarter wave VHF whip from Laird, and the other one is a quarter wave UHF whip from Laird. My question what is the appropriate distance between the two antennas. I have read other threads on this topic and they seem to mention 18 inches as a good distance between antennas. However, currently I have one NMO mount that is used for a temporary setup that I will use for the APX radio which is center mounted. For the other mount following the 18 inch rule, it would end up being mounted on the side of the roof which could cause some issues with polarization and not so great reception.

I have attached two pictures of where I might want to mount the second antenna which is a VHF antenna.
IMG_5825.JPG
This picture shows the VHF antenna mounted in the middle of the car


This picture shows the VHF antenna mounted to the side
IMG_7857-min.JPG
 

cmdrwill

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VHF antenna should be in the middle as that provides the needed groundplane, the other half of the antenna.

The UHF needs at least 14 inches in diameter for it's groundplane.

1/4 wave in all directions for best results. Antenna101.
 

n3obl

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here the other deal. you will only transmit with one at a time anyway so as long you keep about 14" should be ok.
 

mmckenna

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For the other mount following the 18 inch rule, it would end up being mounted on the side of the roof which could cause some issues with polarization and not so great reception.
It isn't a "rule". It works in some applications.

There's a number of variables involved. What you need to be concerned about is the amount of RF getting from the transmitting antenna into the other antenna.

The amount of RF that the receiver will take without causing a failure is usually pretty small, but it varies, and I've never really seen a manufacturer specify any level.

The amount of RF that will couple between the antenna will vary with spacing. The amount of RF that makes it down to the receiver will vary with coax type, length, what the transmit power is, and how much coupling there is between antennas.

Using 50 watt radios and 1/4 wave antennas, I've have not had any issues with using 18-20 inches or so. Your milage may and will vary.

I'd focus more on keeping 2 feet or more between antennas, while making sure each antenna has a suitable amount of ground plane under them. I'd also keep it away from the GPS/XM/Sirus/Cellular/AM-FM good time radio antenna.
 
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