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Author Topic: ?Rig damage from antenna proximity?  (Read 36944 times)
W4CP
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« on: August 21, 2014, 01:06:46 PM »

I'm about to have antennas on (or near) the top of my Honda Pilot for HF (Tarheel II), 2 meters/440 (5/8 wave) and CB (base loaded quarter wave-- for "Why CB?", see end of post.)

The HF antenna will be about 2 feet from the 144/440Mhz antenna and about 5 feet from the CB antenna.  I'll be using an IC-706MKII (at HF only) at about 100 watts and using a separate 144/440 rig. I've had an HF antenna installed near a 144/440MHz antenna in the past with no harm, but I'm worried about the CB since it shares a frequency range with the 706 on HF.

Anyone have any experience with this type of situation?

Long ago, I burned out the VHF tuner of an old (clunk, clunk, rotary dial) TV by using a 40 watt 2 meter rig on an indoor J-pole too close to the rabbit ears. So that makes me a little more paranoid than I might otherwise be.

I'd appreciate the benefit of anyone's experience.

Thanks.


===
WHY WOULD A HAM WANT TO HAVE A CB IN THE CAR?

If you travel a lot via Interstate highways like I do (~100 miles/work day), you will not infrequently encounter traffic jams... very slow traffic or even stopped traffic.  If you want some information about the traffic jam and put out a call on 2 meters or 440Mhz, there is a very small chance that there will be a ham up ahead or in the opposite lane in a position to give you useful information about the traffic situation.

There is a MUCH higher chance of having a traveler or trucker with a CB turned on in their vehicle, in a position to give you useful information.  And hence the desire to have a CB in my SUV.
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K0BG
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« Reply #1 on: August 22, 2014, 06:36:56 AM »

Amateur transceivers aren't TVs. I suspect if there is anything to worry about, it is the CB set. And I wouldn't be too worried about it either. You'll get desense for sure, but only when you transmit.
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KB1GMX
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« Reply #2 on: August 22, 2014, 09:08:29 AM »

This should not be a problem for 2 and 440.

Now if the 706 is on 15, 12 or 10M then I'd consider disconnecting the CB radio
for fear of frying the CB RX front end. They (CB RX) are often not well protected.
The relatively low power 5W CB is no risk to the Ham gear.

My nominal mobile is 6M dedicated SSB radi, 2M 55W FM and a FT817 for hf,
with separate antennas for each.  I keep the 817 off 6 and up to protect the
other radios but on HF there is no interaction. 

If I need traffic I just listen on 11M with the 817 (RX only) as often there is enough
chatter to answer the traffic question without actually needing to transmit.  Whatever
antenna I have mounted is usually good enough to hear trucks withing a few miles. 
Though around here there is enough 2M activity to make that an infrequent need.

Allison
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W8JX
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« Reply #3 on: August 22, 2014, 09:58:08 AM »

This should not be a problem for 2 and 440.

Now if the 706 is on 15, 12 or 10M then I'd consider disconnecting the CB radio
for fear of frying the CB RX front end. They (CB RX) are often not well protected.
The relatively low power 5W CB is no risk to the Ham gear.


I agree but I would not transmit on CB with HF rig connected to antenna. I used to do a lot of HF mobile and never had a VHF/UHF rig effected by a HF rig or in reverse.
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--------------------------------------
Ham since 1969....  Old School 20wpm REAL Extra Class..
K9MAP
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Posts: 3




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« Reply #4 on: August 24, 2014, 12:40:03 PM »

W4CP,
Just curious on how/where you ran power from your battery through the firewall into the cabin of your Pilot.  I have a 2011 Pilot that I'm about to install a mobile radio in, and I'm having trouble finding a place I can run a power cable through the firewall.  I'd much rather do it myself than pay someone else to. 

thanks,
Jeremy N9JTW
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M6GOM
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« Reply #5 on: August 25, 2014, 01:52:08 AM »

Not model specific but on most cars the grommet where the main wiring loom runs into the cabin usually has a nipple on the top you can cut to run a cable through or enough spare space to put a small slit in it to run the cable through. Stiff wire such as a metal coat hanger that you can bend a hook on one end to pull the cable through is invaluable when doing this.
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WV4L
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« Reply #6 on: August 25, 2014, 09:04:11 AM »

Have you done the expanded TX/RX mod on your 706MKIIG? You could at least be able to RX on 27.185 (CB ch. 19) and get info on what is happening. No separate rig or antenna needed. If in an emergency, you could also get help from those monitoring the freq. as well. Some State and local police monitor 27.185 for that very reason. When I go on vacation I always encounter parking garages where my Tarheel HF WILL NOT GO without removing. I take the Tarheel off prior to the trip an use a magmount CB antenna to monitor 27.185 if needed.


73

Wayne C
WV4L
« Last Edit: August 25, 2014, 09:09:26 AM by WV4L » Logged
AA4HA
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« Reply #7 on: August 31, 2014, 03:28:07 PM »

You do not need to do a mod to RX on CB frequencies. Just like you do not need to do a mod to listen to shortwave broadcasts.
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Ms. Tisha Hayes, AA4HA
Lookout Mountain, Alabama
AA4HA
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Posts: 2630




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« Reply #8 on: August 31, 2014, 03:37:19 PM »

Regarding antenna separation;

a 50 watt radio is putting out about 47 dB of power to the antenna.

Five feet away you have another antenna attached to a receiver that is really happy with signals from around -40 to -110 dBm.

Parallel vertical antennas. The coupling between the two is pretty effective at a few feet.

Receivers generally do not like seeing high RF fields being directed into the FET front end.

At five feet what do you think the field strength is?

Sure, there are stages in the receiver that makes the receiver more selective to its frequency but the first RF stage (or pre-amp) usually does not get a selective frequency. It sees (and tries to amplify) everything coming in the front door.

Generally we tried to keep the front end of receivers from seeing more than -40 dBm. Usually above that point we would get excessive AGC action and distortion. One of the other things that gets cooked are PIN diodes.
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Ms. Tisha Hayes, AA4HA
Lookout Mountain, Alabama
W9CMG
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Posts: 47




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« Reply #9 on: September 01, 2014, 09:46:08 AM »

I've got four antennas on a motorhome. A dualband mobile, an APRS station, a CB and a HF screwdriver. I have never had any difficulty with the radios from the proximity of the antennas. In addition, I have a 30' vertical with ground radials that I lay down when stationary. Again, no problems.
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NO9E
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Posts: 886




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« Reply #10 on: September 02, 2014, 06:16:52 AM »

I have 6m quad a few feet operating from 2m quad. Transmitting KW on 6m destroyed 2m front end in IC-7000. 100W was fine. Apparently the input FET in IC-7000 is sensitive to damage. But again, no problems with 100W.

Ignacy, NO9E
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W4CP
Member

Posts: 14




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« Reply #11 on: September 04, 2014, 07:16:09 PM »

W4CP,
Just curious on how/where you ran power from your battery through the firewall into the cabin of your Pilot.  I have a 2011 Pilot that I'm about to install a mobile radio in, and I'm having trouble finding a place I can run a power cable through the firewall.  I'd much rather do it myself than pay someone else to. 

thanks,
Jeremy N9JTW

I cheated. :-)  I paid a company to go through the firewall and put two NMO mounts on the roof.  I went on a couple of Honda websites and asked about going through the firewall (those monster sound system guys have the same problem) and I looked for some of the places they mentioned, and just didn't see anyplace I was comfortable making the cable run.

Because of the sunroof/moonroof/whatever movable roof and the side air bags I wanted to get someone else to install the NMO mounts on the roof and run the coax anyway, so I just added this to the task list.  This is the first time in 40+ years as a ham that I've paid somebody to do some of the work in a mobile installation and I felt bad about it.  I've done all the work on all my previous mobile installations. But things change, and I'm learning to live with it.  ;-)

Sorry I can't be of more help.

Curt W4CP

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W4CP
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« Reply #12 on: September 04, 2014, 07:24:03 PM »

I *could* just listen on the CB freqs during traffic jams and I'm done that in the past with a scanner that covers those frequencies, but I like to be able to ask about specific things regarding the traffic jam incident.  And besides transmitting with a modified 706 on the CB freqs being illegal, we need to set a better example for our CB radio brothers. So I'll have a 4 watt 11 meter transmitter in the mix.

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