Are these antennas too close?

[N4KVE]

Did an install recently with a UHF XPR4550, UHF XTL5000, & 900 XTL5000. All on ham freq's. Laird/Antennex transit low profile antennas. Motorola white teflon coax with NMO mounts. Radios usually transmitting low power. [15-20 watts] Some people are of the opinion that I will soon smoke the receivers on these radios. I am not worried about desensing the receiver on one UHF radio while txing on the other. Any opinions on this?
https://www.facebook.com/photo.php?fbid=10208498618731984&set=gm.1215074591907349&type=3

 

[djpaulino]

Did an install recently with a UHF XPR4550, UHF XTL5000, & 900 XTL5000. All on ham freq's. Laird/Antennex transit low profile antennas. Motorola white teflon coax with NMO mounts. Radios usually transmitting low power. [15-20 watts] Some people are of the opinion that I will soon smoke the receivers on these radios. I am not worried about desensing the receiver on one UHF radio while txing on the other. Any opinions on this?
https://www.facebook.com/photo.php?fbid=10208498618731984&set=gm.1215074591907349&type=3

can you post the the photo for those of us that don't do Facebook?

 

[NC1]

I followed the link, and it says:

"Sorry, this content isn't available right now
The link you followed may have expired, or the page may only be visible to an audience you're not in."

So, your privacy settings on FB are set to where nobody can see it.

 

[N4KVE]

Sorry, while I have the photo on my I-Phone, I am computer challenged & have no idea how to post the photo here. 3 Laird transit antennas 6" space between each of them on the trunk lid. 2 UHF, & 1 900MHz. I would gladly send the photo to anybody who could post it here.

 

[mmckenna]

I've always gone with "at least 1/4 wave" spacing, and that's usually worked well for 50 watt or less radios. Increasing the power will require more spacing.

Obviously that's hard to do on lowband, CB, etc. but CB is usually running only a few watts and the coupling between a really long antenna and a short one reduces it.

Also, the front ends on the Motorola's are going to be a bit more stout that some of the consumer/amateur grade radio equipment. Where I've usually seen issues is with amateur gear or low end commercial radios.

 

[cmdrwill]

Sorry, while I have the photo on my I-Phone, I am computer challenged & have no idea how to post the photo here. 3 Laird transit antennas 6" space between each of them on the trunk lid. 2 UHF, & 1 900MHz. I would gladly send the photo to anybody who could post it here.

Garry, WAY too close, I would not want you to have a burned up frontend receiver stage.

 

[12dbsinad]

Did an install recently with a UHF XPR4550, UHF XTL5000, & 900 XTL5000. All on ham freq's. Laird/Antennex transit low profile antennas. Motorola white teflon coax with NMO mounts. Radios usually transmitting low power. [15-20 watts] Some people are of the opinion that I will soon smoke the receivers on these radios. I am not worried about desensing the receiver on one UHF radio while txing on the other. Any opinions on this?
https://www.facebook.com/photo.php?fbid=10208498618731984&set=gm.1215074591907349&type=3

I would keep the TX power as low as possible. 5-10 watts if possible. Motorola's are pretty robust compared to cheap equipment, however being this close it pumps a lot of RF into the other radios. I would proceed with caution. Try to keep the UHF and UHF radios the furthest apart if at all possible.

 

[prcguy]

Are the antennas arranged where the 900 is in the middle with the two UHFs on either side? That would place the UHF ants 12" apart (1/2 wavelength) and the 900 antenna would be invisible as far as VSWR affects go. The 900 antenna would be 1/2 wavelength away from its nearest neighbor.

I see no interaction problems between the UHF radios and the 900 radio due to front end filtering, low pass filters on transmitters, etc. The coupling between the two UHF radios is a little concerning but at the power levels you have I would not expect any problems. I've done some pretty scary experiments in the past with 100w radios and antennas nearly touching and have never damaged a radio in the process.
prcguy

 

[N4KVE]

Are the antennas arranged where the 900 is in the middle with the two UHFs on either side? That would place the UHF ants 12" apart (1/2 wavelength) and the 900 antenna would be invisible as far as VSWR affects go. The 900 antenna would be 1/2 wavelength away from its nearest neighbor.

I just rearranged the antennas so that's how it is now. This would be the safest way. I only transmitted a few times with the 2 UHF antennas next to each other. Thanks for the tip.

 

[romanr]

Gary's Picture

Gary, I hope you don't mind - I've attached your picture for you.
Best Regards,
RR

 

[jim202]

You would have been better off with the UHF antennas on each side of the trunk with the 900 in the center. Now that you have the holes there, you will have to live with it.

 

[riccom]

I think a roof mounted setup with a good space of 18 inchs may have been better or what jim202 pointed out, not only for rf feedback but rf footprint.


Sent from my iPhone using Tapatalk

 

[N4GIX]

That is the "old picture." The center UHF and the 900 MHz antennas have been swapped around in the new arrangement.

I also agree with the suggestion to use braided wire at each hinge to better bond the trunk deck with the rest of the car. Doing so will insure the entire vehicle acts as a ground plane.

 

[Golay]

Agree

You would have been better off with the UHF antennas on each side of the trunk with the 900 in the center. Now that you have the holes there, you will have to live with it.

Even with the holes there, I would still do like Jim says and space the antennas out across the trunk, like this.
And get a couple of the plastic plugs for the holes that you can buy at any hamfest.

 

[iamhere300]

Did an install recently with a UHF XPR4550, UHF XTL5000, & 900 XTL5000. All on ham freq's. Laird/Antennex transit low profile antennas. Motorola white teflon coax with NMO mounts. Radios usually transmitting low power. [15-20 watts] Some people are of the opinion that I will soon smoke the receivers on these radios. I am not worried about desensing the receiver on one UHF radio while txing on the other. Any opinions on this?
https://www.facebook.com/photo.php?fbid=10208498618731984&set=gm.1215074591907349&type=3

Chuckle. I have seen this picture lately! What a comment string it got going.

 

[prcguy]

Adding braid to ground a trunk lid will very slightly help a CB/HF antenna achieve more ground plane but not for VHF and above. The VHF/UHF antennas will never know the braid or the rest of the vehicle exists and the only useable ground plane will be the trunk lid.
prcguy

That is the "old picture." The center UHF and the 900 MHz antennas have been swapped around in the new arrangement.

I also agree with the suggestion to use braided wire at each hinge to better bond the trunk deck with the rest of the car. Doing so will insure the entire vehicle acts as a ground plane.

 

[12dbsinad]

Adding braid to ground a trunk lid will very slightly help a CB/HF antenna achieve more ground plane but not for VHF and above. The VHF/UHF antennas will never know the braid or the rest of the vehicle exists and the only useable ground plane will be the trunk lid.
prcguy

Reminds me of the braid ground straps on the Crown Vic trunk lids that use to last 6 months before they broke off!

 

[N4KVE]

Took a VOM to my car & put 1 lead to the trunk lid, & the other to the car body. No resistance at all, so they are somehow connected. The 1 radio that was there before I added the others is doing very well with the low profile antenna. 60 miles at the 25 watt setting.

 

[mmckenna]

Took a VOM to my car & put 1 lead to the trunk lid, & the other to the car body. No resistance at all, so they are somehow connected. The 1 radio that was there before I added the others is doing very well with the low profile antenna. 60 miles at the 25 watt setting.

Keep in mind that DC ground and RF grounds can be different things. Also, while driving, vibrations may cause the mechanical ground connections of the trunk lid hinges to change.
Braid at the two forward corners can be a good idea if you suspect an issue. If it's working fine as is, then don't worry about it. The trunk lid is a big enough ground plane at UHF and 900MHz.

 

[N4KVE]

The trunk lid is a big enough ground plane at UHF and 900MHz.

Yup, that's all I got.