Antenna Interfering w/ Transceiver on VHF TX - Poor case shielding on Chinese 10W Mini-Mobile?

[chiwititsara]

I have a small Chinese 10 watt mobile radio (Leixen VV898 or clone) mounted on the inside of a windowsill and an antenna directly outside the same window. The radio is powered by a ~13.8 volt DC switching regulated power supply connected by a few feet of 10 gauge copper wire. The radio has a plastic casing on its top and sides and an aluminum heatsink for its base.

I am having an issue where transmitting (VHF-high) at 10 watts causes the radio display to dim and measured voltage between the power supply and the radio drops dangerously low (8 volts or less at times, causing the display to dim or turn-off entirely).

TX'ing at 5 watts causes much less of a problem.

I believe the source of my problem is antenna RF entering the radio (and not DC power / power line interference) because the further I move the radio from the antenna, the less voltage drop I observe. There is no correlation with proximity to the DC power supply nor routing of the DC power lines.

With the radio installed right-side up (as designed) and separated ~10 feet horizontally / ~4 feet vertically from the antenna, the interference lessens but is still unacceptable, with voltage dropping 15% or more from the designed operating voltage.

Interestingly, at this same distance - and even much closer - I have found a simple solution that almost entirely mitigates my problems: Orienting the radio with the aluminum heatsink (base) facing the antenna and 'shielding' the transceiver's internals almost entirely cures the issue.

1. Does it seem correct that my problem is RF from the transceiver antenna entering the transceiver and causing my problems?

2. If so, what is a practical solution to prevent this that does not require entirely re-locating the antenna/radio beyond ~15 feet separation (any further and I would have to move to another room)?

Are there any recommended sheet metal enclosures / radio cabinets I might use to enclose the transceiver and block RF from entering the unit? I don't have a welder or scrap sheet metal just sitting around here but perhaps I could re-purpose some sort of metal enclosure designed for other electronics. Heat is not a real concern given the low duty cycles and low wattage I am using.

 

[RFI-EMI-GUY]

have you tested with the radio terminated into a dummy load? What model power supply are you using?

 

[mmckenna]

Good idea above.

Usually this indicates that the power supply is undersized and is starving the radio for power, but since it lessens when moving away from the antenna, it sounds more like RF getting into the power supply.

Still, checking DC voltage at the power supply when transmitting is a good troubleshooting step.

1. Does it seem correct that my problem is RF from the transceiver antenna entering the transceiver and causing my problems?

Or, getting into the power supply through the radio and power wiring.

2. If so, what is a practical solution to prevent this that does not require entirely re-locating the antenna/radio beyond ~15 feet separation (any further and I would have to move to another room)?

Well, getting the antenna away from the radio is what you need to do. Since we don't know exactly what the cause is, making a recommendation other than what you've already noticed would be guesswork.

High RF getting into a cheap plastic shell radio is a possibility.
High SWR may be an issue. When you hold on to the radio when you move it away from the antenna, capacitive coupling to your body could be changing the groundplane for the antenna

Knowing more about the antenna, cable, and location would be helpful.

Are there any recommended sheet metal enclosures / radio cabinets I might use to enclose the transceiver and block RF from entering the unit? I don't have a welder or scrap sheet metal just sitting around here but perhaps I could re-purpose some sort of metal enclosure designed for other electronics. Heat is not a real concern given the low duty cycles and low wattage I am using.

I'd try some aluminum foil, first. See if that helps. That might help narrow down if it is high RF getting into the radio/power supply.
If that fixes it, you can go to a hardware store and check their electrical department for a large enough metal junction box to mount your radio in. It'll have knockouts for routing wiring.
But that may not be enough. You may need to install some chokes on the DC power cable, microphone/speaker cable, etc.

High SWR (poorly tuned/unsuitable/poor location) on the antenna system can cause weird issues. Getting the antenna up in the clear and away from the window/screen/window frame/home may be required.

 

[chiwititsara]

have you tested with the radio terminated into a dummy load? What model power supply are you using?

Yes, and I should have mentioned this. When connected to a 50-ohm, 50(?)- watt rated UHF-connector dummy load, I have no voltage drop - nada.

...
Knowing more about the antenna, cable, and location would be helpful.
...

Many good thoughts from the always helpful mmckenna here... Have tried a number of 10-watt rated helical 1/4 waves from Smiley, a collapsible 1/2 wave, and some larger non-helical 50/100-watt rated 1/4 and 5/8 wave's. Extent of interference does seem somewhat dependent on the antenna...

For coax, I have experimented with everything from generic RG-213 to Times Microwave RG8X, LMR240UF, and LMR400 - generally just whatever I happened to have easily at hand in different lengths. Couldn't tell any real difference but each were of different lengths as I tried moving to different distances.

I'd try some aluminum foil, first. See if that helps. That might help narrow down if it is high RF getting into the radio/power supply.
If that fixes it, you can go to a hardware store and check their electrical department for a large enough metal junction box to mount your radio in. It'll have knockouts for routing wiring.

Yes, the foil idea hit me shortly after I posted this. Will try... My hunch is that this is not a P/S or power line issue but the incredibly-unfortunate choice of plastic by a budget Chinese manufacturer for their case material. This will be a good test. Electrical junction boxes would be a cheap enclosure...or perhaps a .30 cal ammo can on its side with lid removed.

High SWR (poorly tuned/unsuitable/poor location) on the antenna system can cause weird issues. Getting the antenna up in the clear and away from the window/screen/window frame/home may be required.

This I think is also a likely contributing factor, possibly focusing the RF in some weird direction. In the clear, all of these antennas (on their mounts) have looked good on my NanoVNA. But the mount is now quite close to a metal framed window and I am well-aware that this is far from optimal.

 

[jeepsandradios]

I realize you know this but a CCR is what it is. They are not designed well and either they work or dont. Moving the antenna sounds like the simple solution.

 

[RFI-EMI-GUY]

Try some ferrite beads/chokes on the power supply leads.

 

[FKimble]

If there is enough RF getting back into your radio/power supply, it's also getting in your head! An antenna too close to the radio will always dump RF back to the radio. A well built radio may handle it. Most radio's are not designed to put an antenna right next to it. Do it right move the antenna farther away.

Frank

 

[mmckenna]

Many good thoughts from the always helpful mmckenna here... Have tried a number of 10-watt rated helical 1/4 waves from Smiley, a collapsible 1/2 wave, and some larger non-helical 50/100-watt rated 1/4 and 5/8 wave's. Extent of interference does seem somewhat dependent on the antenna...

I think that's a good clue, right there.

Do you have a proper ground plane under the antenna?
1/4 wave and 5/8th's wave antennas need to have a ground plane under them to work properly.
A 1/2 wave antenna will work better with a ground plane, but will also result in a lot of RF flowing on the outside of the coax without one. It uses the coax shield as part of the antenna system.

Sounds like RF getting back into the radio and maybe the power supply. Wrapping things in foil probably won't help, and won't fix the real issue you have.

Get a suitable base station antenna, or at least a mobile antenna with a base station adapter.