Dual Band Antenna Issue

[N9JIG]

On my IC-706MK-IIG mounted in my van I tried to use a Comet rubberized dual band antenna (rubber flexible whip) that works just fine with my Kenwood TMD-700 as well as other radios, even with the same mount and coax. The 706 will not work with this antenna, the SWR is atrocious and RX sensitivity is almost non-existent. I was trying to see what was going on and as I removed the PL259 from the radio as soon as the ground disconnected and just the center pin of the PL259 was touching the SO239 center the receiver came alive on a NWS channel.

I tried a few other antennas and found that the radio worked great on 2M with a plain old 1/4 wave NMO whip, same for 440. I found a dual band MaxRad that worked well too.

I used an ohm meter to find that the Comet and a couple other dual band antennas have show a dead short across the center pin and thread on the NMO while the 1/4 wave does not of course. I also checked the continuity of the coax and it was fine.

Is there possibly a short on the antenna jack of the 706 that could cause this or is this normal? I find it weird that the Kenwood (and the IC-2800 that preceeded it) work great with this Comet dual band antenna.

I also tried my Diamond 8900 and it worked fine, and also shows open between the center pin and collar.

The Comet dual band also works great on my BCD996T, even better on VHF than a 1/4 wave.

 

[nd5y]

I think what you described may be the same problem I had with a Comet SBB-7 NMO antenna that appeared shorted when installed on the NMO mount.

I thought the radio was shorting it at first but then found out the cause of the short was the center contact of the NMO mount touching the antenna base around the center pin.

The center contact on the NMO mount has a larger diameter than the "hole" in the center of the antenna where the spring loaded center pin is located. It also sticks up too high. When I screwed the antenna down all the way on the mount, it shorted out. If I loosened it a little the short went away.

I fixed it by filing down the center contact on the NMO mount and also carefully ground down or countersunk the hole around the antenna center pin with a dremel tool.

 

[N4UMJ]

Sure sounds like maybe a bonding/grounding problem with the 706MKIIG that was not a problem with the former rigs. Are any of these mounts magnetic? The degree of magnetic coupling and antenna positioning can cause some strange SWR results.

Lots of times here in the shack I will hook up the transceiver red power lead and before I know it the radio powers on without the negative lead even being connected. This is due to the ground being provided through the antenna ground.

If your mobile ground has become intermittent or loose this alternate ground path(coaxial) may be contributing to the variance in SWR between known good antennas.

I would closely inspect your power lead connections (especially the ground lead) wherever they make contact with your electrical system. Corrosion around the battery terminals can be concealed easily and I try to clean mine up on occasion.

Also there is a bonding strap point on the rear of the 706MKIIG that you might could utilize but I normally try to only provide one grounding point through the power lead when I am doing a mobile install.

Good Luck,
Ed
N4UMJ

Is there possibly a short on the antenna jack of the 706 that could cause this or is this normal? I find it weird that the Kenwood (and the IC-2800 that preceeded it) work great with this Comet dual band antenna.

I also tried my Diamond 8900 and it worked fine, and also shows open between the center pin and collar.

The Comet dual band also works great on my BCD996T, even better on VHF than a 1/4 wave.

 

[n8emr]

Dont overlook the 706, My 706 had a similar issue on the HF port. The pins on the so239 are set back a liitle more in the sock than some connectors and the crimped jumper cable I was using wasnt touching. Switching to a non-crimped connector solved the problem.