Front end overload?

[nobody]

I just installed a Kenwood TM 281A radio in my Dodge Ram club cab. I all ready had a Galaxy DX949 CB in the truck. The CB's antenna is an 102" whip mounted on the driver's side behind the cab. The Antenna for the 2 meter radio is a 5/8th wave Laird mounted in the center of the cab with a NMO and spring mount.
The problem is if I key up the Kenwood the signal meter on the CB pegs out rather or not the radio is on or off. Today I noticed on some frequencies on the 2 meter radio I get a little interference when I key up the CB radio. Not much but enough to notice. Should I be concerned? Any ideas on a fix?
FWIW I plan to move the CB antenna once I get a tool box. I will then mount that antenna (for the CB) to the rear and center of the tool box. I also plan to add a 70cm radio to the mix also with more radios on down the road. The console I made has room for one maybe two more radios if they are small enough.

 

[Shockwave]

The problem is a 5/8 wave 2 meter antenna is very similar to the electrical characteristics of a base loaded CB 1/4 wave. They are coupling together strongly even though they are far apart in frequency because they are so close in proximity. If it were me I'd probably use a coaxial stub or trap to null out the VHF signal on the CB. I do have a simpler fix for this problem. Install a low pass TVI filter inline with your CB antenna. This will provide 60 to 80 db of attenuation to the transmitted VHF signal in the CB radio.

 

[nobody]

Thanks Shockwave.

I thought the TVI filters only prevented the radio from causing interference to other devices. I didn't think it would help with the reception side of it. Any suggestions on a particular unit?

BTW I forgot about the 5/8th wave antenna being close to a 1/4 wave CB antenna until I got it home and saw the base loaded CB antenna on the other truck. I then suspect I might have some issue but figured since the radios were so far apart in frequency I would still be good to go. Not to mention I've seen other install pictures of mobile setups and saw multiple antennas, varying in band(s) REAL close to each other and no comments in regards to interference. Mine are about 3 foot apart at the moment.
For future reference would it have been better to go with a 1/4 wave 2 meter antenna for this setup? When I was looking for mine I wanted something a little wide banded and with some gain to it. Another reason I like the 102" whip for the CB.

 

[Captain Kilowatt]

Pretty much any decent LPF would work but I advise to use one meant for amateur radio and not just CB service as they tend to be of much better quality. As for it working on receive, I forgot and left my filter inline with my FT-857 and couldn't figure out why I could not hear the local 6m repeater that was normally nearly full scale. Then I saw the SWR indicator flashing when I transmitted and realized I still had the filter inline. I removed the filter and the repeater was back to nearly full scale. LP filters are bidirectional and work either way.

As for the 2m 5/8 being near a loaded CB 1/4 wave I disagree. A 2m 5/8 wave antenna performs rather nicely on 6m as a slightly loaded 1/4 wave but is no where near resonant on the CB band.

 

[nobody]

Pretty much any decent LPF would work but I advise to use one meant for amateur radio and not just CB service as they tend to be of much better quality. As for it working on receive, I forgot and left my filter inline with my FT-857 and couldn't figure out why I could not hear the local 6m repeater that was normally nearly full scale. Then I saw the SWR indicator flashing when I transmitted and realized I still had the filter inline. I removed the filter and the repeater was back to nearly full scale. LP filters are bidirectional and work either way.

As for the 2m 5/8 being near a loaded CB 1/4 wave I disagree. A 2m 5/8 wave antenna performs rather nicely on 6m as a slightly loaded 1/4 wave but is no where near resonant on the CB band.

Your right I got that mixed when talking about 5/8, 1/4 etc and was thinking about the Little Will I mounted on the other truck at the time. :headbang

 

[Shockwave]

As for the 2m 5/8 being near a loaded CB 1/4 wave I disagree. A 2m 5/8 wave antenna performs rather nicely on 6m as a slightly loaded 1/4 wave but is no where near resonant on the CB band.

I didn't mean to suggest the 5/8 wave 2 meter antenna would give a good match on the CB. Just that a 2 meter antenna with a longer then 1/4 wave whip is inherently going to couple more VHF energy to the long CB antenna. Might even be a few pf of capacitance between them.

For those who have seen other antennas covering different bands that were this close without problems. They were probably using commercial 2-Way equipment like Motorola. They have extensive front end filtering to reject received RF just 500 KHz away from the tuned frequency. Typically used in Police, Fire, and Ambulance vehicles where several priority conversations may be taking place on different frequencies from the same location.

The realistic goal I would look for in this case would be to stop the CB front end from taking such a dose of VHF energy that it moves the S meter when the radio is off. The TVI filter should insure that when the CB is on and the 2 meter is keyed, it will not damage the CB front end. You may still have interference on either receiver when the opposite transmitter is keyed. That would be harder to correct.

 

[Captain Kilowatt]

Agreed that the TVI filter would likely help the interference to the CB. Like you said, most commercial gear, especially the older crystal controlled gear, has a MUCH better ability to reject out of band or even in band interference.A lot of gear has what is called a helical resonator which is a series of highly tuned circuits on the front end.Way back about 20 years ago I pulled the helical resonator out of a Marconi DT-56 business band VHF mobile radio and put it across my cable TV line to ground. It was so high Q that I was able to filter out the pulsed scramble signal from a pay TV channel on the old analog cable TV system without loosing signal quality. I got The Movie Network and Family Channel without paying due to the quality of the resonator circuit from a receiver's front end.

 

[nobody]

Thanks for the advise folks. Will try the above.

 

[nobody]

Update

I ordered the "MAGNUM DF-3000" (RF Limited) from H&Y and just tried it on the radio. With the filter installed in line with the CB antenna, the CB could not break squelch on the Kenwood and the Kenwood, when keyed up on full power (65 watts) would only cause a very, very small bump (hardly noticeable) on the CB's signal meter and nothing more. With the CB radio off it's meter would not move at all anymore.
However if the Kenwood was receiving another station and I keyed up the CB at the same time, it would cause a little hash to come through with the received audio on some frequencies. Other than that the other station still sounded great.

Thanks again for the help!

 

[nobody]

I spoke to soon. I got a jumper cable and did some more testing. On some frequencies the CB, with the filter installed, is still causing interference on the Kenwood even with the squelch level turned up.:angry:

 

[Robb]

Here are some thoughts . . .

That Magnum filter is a Low Pass filter, which is designed to take out 2nd and also 3rd harmonics of the fundamental freq you are on. But if that Galaxy radio has more than 100% modulation; then it is putting out IMD distortion. Not fundamental harmonic distortion. Two different types of distortions that a CB radio can generate. The solution would be to turn the mic modulation down to less than 100%. TO experiment to see if this is the cause of the problem you are experiencing; then try turning the mic gain way, waaay down on the Galaxy radio. Way down; then retest.

I run a Diamond NR-770 and a Sirio Z-180 and haven't had the problems you are having. They are both magnet mounts as well and less than 3 ft apart.

Let us know . . .

 

[Captain Kilowatt]

I spoke to soon. I got a jumper cable and did some more testing. On some frequencies the CB, with the filter installed, is still causing interference on the Kenwood even with the squelch level turned up.:angry:

Ideally you should install the filter as close to the transmitter as you possibly can. No cable is too short for this connection and a few inches is great. Hard to do unless you make your own but it is the best method.There is a possibility that some slight desense may still occur. Desense is when a receiver in one radio is temporarily desensitized by a strong radio signal on an unrelated frequency. This happens because the receiver cannot handle the shear strength of the signal and causes the receiver's first amplifier stage to overload. Normal sensitivity is restored as soon as the strong signal goes away.

 

[nobody]

Tried the adjusting the power and mic gain on the CB radio. Neither appeared to make any difference.
However this evening the interference didn't appear as bad for some reason. Didn't change anything.

I also noticed as I changed the channel on the CB radio it also effected different frequencies. For example channel 34 would cause interference on the "NFL simplex" frequency. If I changed the channel to 35 I could hear some on ~155.790 Mhz (receive only, no mods have been done to the Kenwood). Another channel would cause some on or about 146.00 Mhz etc. FWIW with CB keyed up I could hear what sounded like a distant talk radio show and commercials on the above frequencies.

If I get time tomorrow I'll play with it some more. If I can get some more PL259 connectors I'll try shortening the jumper cable. I actually thought about, before I got the filter, using a double male connector to attach it to the radio. But the thing is to freaking big for that.