Antenna Spacing on vehicle

[KG5HHS]

Hello,
I have a 2010 ford explorer with a sunroof. I currently have a NMO 5/8wave VHF antenna above the very back dome light. I'm wanting to put a VHF/UHF antenna for the second radio above the middle row dome light (about a foot apart). is this going to be enough spacing between the two? the VHF radio is a Motorola 110watt radio, the second VHF/UHF Radio is just a little 25 watt radio.

 

[cmdrwill]

You need a lot of separation with the Hi Pwr Spectra. AND plenty of ground plane area on all sides of the antenna, 1/4 wave minimum in all directions. .

 

[wyomingmedic]

Yup. Shoot for at least 1/4 wavelength for the lowest frequency you will be using. In this case it would be about 20 inches.

BUT, given that you will have a VH110 watt radio and a VHF (I assume ham rig), I would shoot for as much distance as you can muster.

 

[PACNWDude]

That Spectrais going to radiate into the other radio. You best bet would be to put the lower power antenna on a front fender NMO mount. Large separation is best. 110 Watt radios will overpower a 25 Watt unit easily.
I used to install Spectras into small work boats. A previous installer mounted the antenna and radio next to their VHF marine radios at 25 Watts. Then they wondered why they were losing marine radios. I moved the antennas as far apart as possible and mitigated the problem.

 

[KG5HHS]

thank you for the replies. I think Ill mount on the fender.

 

[Project25_MASTR]

I run a 110W Astro Spectra on my pickup…while a commercial front end could handle 19" of separation…the receiver on an amateur grade radio will be lucky to handle a full wave of separation (in band), UHF should be fine at 1/2 wave lowest.

I used to run an Icom F221 for UHF and a Yeasu FT2900 on my DD. With a 5/8 over 5/8 on UHF, desense from other UHF radios used within 30 feet would lock the thing out. Swapped to a PCTEL wide band knob…that problem went away. Had about 4 feet of separation between the Yaesu and Icom and at high power with a 5/8 wave antenna the Yaesu would occasionally desense the Icom.

My current pickup…triband quarter wave.

 

[jim202]

One point the might be considered here is to add a pair of hot carrier diodes back to back at the input to the first active RF section of the receiver. Make sure you have a series resistor to limit the current. Isolate the diodes with a cap going to the active stage. One end of the hot carrier diodes to ground and the other end to the junction of the series resistor and the coupling cap.

This mod will probably require the circuit land printed circuit be cut to allow the parts to be added.

I had to do this mod some years back to a VHF GE Mastr II in my truck that had the front end FET blown out several times from a high powered UHF sounding radar at the field site I worked at. The first time it happened, I just wrote it off as something I couldn't explain. When it happened the second time, I took a long hard look at what may be causing the blown front end RF amp. Even with the helical front end, there was enough energy hitting the gate of the FET to blow it out.

After the addition of the hot carrier diode mod, never had another problem with the front end RF FET.

In my current truck, I have a VHF mid power XTL2500 and a high power VHF Spectra set to about 75 watts out. The antennas are on opposite sides of the roof over the back seat. Also have a low band Syntor X9000 set at about 75 watts with the antenna in the middle of the roof.

So far have not had any receiver problem with any of the radios. Goes to show that a good receiver design can survive high RF levels coming into these radios. As well as the GE Mastr II was designed, that receiver could not cut the mustard under the same conditions.

Jim

 

[byndhlptom]

Vehicle antenna spacing

From a practical perspective, you will not get any usable isolation on a vehicle roof. Just try to get 1/4 wave for each antenna and space them at least 1/4 wave from each other. There will be interaction for some combos no matter what you do.

I've had as many as 9 antennas on a roof, just learned to live with the desense and interference...

$.02

 

[cmdrwill]

I've had as many as 9 antennas on a roof, just learned to live with the desense and interference... $.02

And I have replaced way too many RF frontend, receiver, transistors due to antennas too close.

 

[Voyager]

He said he lived with the desense. He didn't say the receiver recovered from that desense. :wink:

(translation: some receivers now have permanent desense due to blown front ends)

 

[byndhlptom]

Vehicle Antenna spacing

I have had no RX damage, used lowest power needed for comm. Had 10mtr/11mtr/6mtr/2mtr/.75mtr + 2 scanner antennas all on roof. Just accepted that when TX'g on one band, I may not hear other freq.....

$.02

 

[Rred]

A fender mount may be less than ideal since your ground plane will still be the roof, well above the antenna's base. Something that might be worth considering is to abandon (cap off) the existing NMO, and install two new ones, each located on the roof above the "B" pillar on the truck. That gives each one some separation (four? feet apart) and a decent ground plane with the B pillar being used as a compromise for part of it.

Yes, it is a gamble on two holes in the roof, but perhaps worth buying or borrowing two NMO-magmount bases to try out the "stereo" location, and if it works, then drill the roof.

 

[kayn1n32008]

A fender mount may be less than ideal since your ground plane will still be the roof, well above the antenna's base.

With a fender mount antenna, the ground plane will be the hood/fender. In essence you will have half a ground plane, which will make the antenna somewhat directional. Vs. Being centre mount on the roof.

 

[Rred]

I can't explain the physics to you, but I think QST covered that question this year, and did explain why having the base of your antenna located firmly below the ground plane was a bad idea. That's why the performance on an antenna "to the side" but physically located ON the roof and extending upwards from the ground plane, would beat having a quarter panel mount (fenders are just up front, I'm sure the OP meant a quarter panel mount in the rear, as I did) which probably would put 2 to 2-1/2 feet of the whip transmitting "into" ground instead of above it.

 

[cmdrwill]

He said he lived with the desense. He didn't say the receiver recovered from that desense. :wink:
(translation: some receivers now have permanent desense due to blown front ends)

Exactly, .

 

[wa1nic]

I installed both an Alinco 2 meter radio and an ICOM 50 watt VHF commercial radio in my pickup a couple of years back.

The antennas were 1/4 wave hips about 6 feet apart.

First I tuned the Alinco 2 meter radio, then I tuned the ICOM VHF radio.

Then I went back to the Alinco and found that I had blown out the T/R switch diodes in it. That kinda sucked.

I wound up going back to 1 radio, programming ham repeaters into the ICOM commercial radio. I also added a scanner with a homebrew front end protection network (clamping diodes and a saturating transformer).

 

[cmdrwill]

Hammy radios do not have good robust antenna switching or frontends.

Two 220 Alinco mobiles had to go back to Remtronics service station, one a second time they replaced the whole main board. Why? High SWR from a Ed Fong 220 "J pole" antenna 'blew out' the receiver frontend. I will say Remtronics has EXCELLENT service.