Help VHF antenna, transmit with no receive

[jwreich]

Hello all,

I'm a new ham who has been trying to learn and get my rig set up. I'm having issues with my HT (Retevis RA-89) receiving VHF when connected to the antenna mounted to my truck. I can transmit and receive when using a Diamond RA707, but audio is scratchy. I do not have a proper mobile radio yet.

Setup: My Dodge truck has a NMO finder mount with a Comet NMO coax connected to a Comet CA-2X4SR. SWR has been checked and is reading 1:1 on both VHF and UHF repeater frequencies. I analyzed the antenna with a NanoVNA and get similar results.

I get excellent audio reports on our local UHF repeater and I'm able to both RX and TX without issues. When I switch to our various VHF repeaters, I get good audio reports, but I can not hear responses. As soon as I attach the RH707, I can receive audio without issues. This confirms my code plug is correct.

I can receive NOAA (VHF) when I'm connected to the truck antenna.

Current continuity tests on the Coax to rule out a short:
- Center PIN of NMO to Ring - No continuity/ short at the connector
- Center PIN of NMO to center conductor at Radio - Continuity though Coax to radio
- Ring of NMO to Ring at Radio - Continuity ground from antenna to the radio
- Center PIN of NMO to Ring at Radio - No Continuity/ no short
- Ring of NMO to Center conductor at Radio - No Continuity / no short
- Center conductor to ring at Radio - No Continuity/ no short
- Ring of NMO to Finder Mount - Continuity (grounded)
- Finder Mount to known ground - Continuity (grounded)
- Finder Mount to radio ring connection - Continuity (grounded)


I've tested with multiple copies of the RA-89, Ailunce HD-1, and Quansheng UVK8. All exhibit the same oddities so I don't think it is the Radio. I have a great setup, but I have some problems.

Can anyone provide any suggestions for additional testing or configuration options to help me make successful connections with our VHF repeaters.

 

[merlin]

Antennas don't know the difference between TX and RX. Often it doesn't take much power to hit a repeater.
You do have antenna problems, likely the mounting location makes them directional.
You may be hitting a repeater that has poor TX coverage at your location.
Checking your antenna with a Nano VNA, no need to continuity check if your SWR shows OK, your antenna is good to go.
Don't forget about line of sight. Buildings, trees, and terrain can cause odities with both reception and transmission.
My best suggestion is mounting the antenna at the roof center or close as you can get.
When you check your antenna, keep clear of trees and buildings.
Try another location, like a Walmart parking lot. I do that, I loose 1 repeater and gain two others. That is with my UV-K8.
Hey, keep your squelch setting just into quiet with no signal.

 

[G7RUX]

Hello all,

I'm a new ham who has been trying to learn and get my rig set up. I'm having issues with my HT (Retevis RA-89) receiving VHF when connected to the antenna mounted to my truck. I can transmit and receive when using a Diamond RA707, but audio is scratchy. I do not have a proper mobile radio yet.

Setup: My Dodge truck has a NMO finder mount with a Comet NMO coax connected to a Comet CA-2X4SR. SWR has been checked and is reading 1:1 on both VHF and UHF repeater frequencies. I analyzed the antenna with a NanoVNA and get similar results.

I get excellent audio reports on our local UHF repeater and I'm able to both RX and TX without issues. When I switch to our various VHF repeaters, I get good audio reports, but I can not hear responses. As soon as I attach the RH707, I can receive audio without issues. This confirms my code plug is correct.

I can receive NOAA (VHF) when I'm connected to the truck antenna.

Current continuity tests on the Coax to rule out a short:
- Center PIN of NMO to Ring - No continuity/ short at the connector
- Center PIN of NMO to center conductor at Radio - Continuity though Coax to radio
- Ring of NMO to Ring at Radio - Continuity ground from antenna to the radio
- Center PIN of NMO to Ring at Radio - No Continuity/ no short
- Ring of NMO to Center conductor at Radio - No Continuity / no short
- Center conductor to ring at Radio - No Continuity/ no short
- Ring of NMO to Finder Mount - Continuity (grounded)
- Finder Mount to known ground - Continuity (grounded)
- Finder Mount to radio ring connection - Continuity (grounded)


I've tested with multiple copies of the RA-89, Ailunce HD-1, and Quansheng UVK8. All exhibit the same oddities so I don't think it is the Radio. I have a great setup, but I have some problems.

Can anyone provide any suggestions for additional testing or configuration options to help me make successful connections with our VHF repeaters.

An interesting issue, no doubt.

When you have the nanoVNA attached and the Comet antenna fitted, what happens to the VSWR when you put your hand near or touch the antenna?

This sounds rather like there is some sort of issue with your installation but what it is may be harder to pinpoint without more details.


If you are comfortable doing so, have the nanoVNA display S11 on a Smith Chart and see if you can show what the plot looks like...I am a little suspicious of the 1:1 VSWR for both VHF and UHF as this seems a little *too* good and I would expect to see something like 1.1: to 1.3:1 in a real antenna system.

 

[ramal121]

Indeed that's odd. Don't discount RFI that only shows up on the VHF band. LED lights, engine noise, car chargers for cell phones, etc can all be suspect and throw enough hash out that will wreck your receive. Good luck on your troubleshooting.

 

[mmckenna]

I agree with Ramal. Band specific noise might be causing an issue. USB adapters, after market LED lights, other electronics. Also, some Dodge vehicles are know to have really bad RFI issues.

Also, don't rule out a damaged phasing coil in the antenna. Some of those ham grade antennas are built pretty poorly and may not survive a strike. The stacked elements may result in a low SWR, but poor performance. Since you can switch back to the other antenna and it works, that seems to suggest an antenna issue. Maybe replace the ham grade antenna with something like a Larsen NMO-2/70 if you find the Comet is at fault.

 

[jwreich]

Indeed that's odd. Don't discount RFI that only shows up on the VHF band. LED lights, engine noise, car chargers for cell phones, etc can all be suspect and throw enough hash out that will wreck your receive. Good luck on your troubleshooting.

Thank you. All testing is performed with the truck off and sitting still. Now power to accessories.

 

[jwreich]

An interesting issue, no doubt.

When you have the nanoVNA attached and the Comet antenna fitted, what happens to the VSWR when you put your hand near or touch the antenna?

This sounds rather like there is some sort of issue with your installation but what it is may be harder to pinpoint without more details.


If you are comfortable doing so, have the nanoVNA display S11 on a Smith Chart and see if you can show what the plot looks like...I am a little suspicious of the 1:1 VSWR for both VHF and UHF as this seems a little *too* good and I would expect to see something like 1.1: to 1.3:1 in a real antenna system.

I'll retest while moving my hand near the antenna. I agree it seems like an install issue I'm chasing down. I did add RF beads and it did not change the results.

I will research the Smith Chart so I understand what I'm doing.


I have a Comet mag mount that I will test with the antenna to see if I have a different experience. Thanks/JR

 

[jwreich]

An interesting issue, no doubt.

When you have the nanoVNA attached and the Comet antenna fitted, what happens to the VSWR when you put your hand near or touch the antenna?

This sounds rather like there is some sort of issue with your installation but what it is may be harder to pinpoint without more details.


If you are comfortable doing so, have the nanoVNA display S11 on a Smith Chart and see if you can show what the plot looks like...I am a little suspicious of the 1:1 VSWR for both VHF and UHF as this seems a little *too* good and I would expect to see something like 1.1: to 1.3:1 in a real antenna system.

Ok, I tried to read up on Smith Charts and all of the data included.

When I touch the NMO base of the antenna, not the physical mount, the impedance and SWR numbers jump all over the place. I am going to try and get some screen shots.

When I touch the physical mount in the truck, nothing changes.

When I touch the antenna above the NMO base, nothing really changes.

I did notice the the SWR jumps up when I move my hand from the connection jumper of the NANO VNA.

I also have the same antenna on a Comet Mag mount. It exhibits the exact same lack of RX characteristics when positioned in the middle of the roof. When testing it with the NANO VNA, it does not fluctuate in SWR regardless of my hand placement.

I'm going to put a call out to some elmers in the area for assistance.

 

[jwreich]

I did another round of testing with a Signal Stick on a Signal Stick magnetic mount terminated BNC Male.

The good:
- SWR was consistent across the VHF and UHF bands.
- impedance was more consistent across the spectrum
- I was able to get a squelch tale response from one of the local VHF repeaters (9 miles)
- the Smith chart looked very consistent with what I've seen on the internet.
- Excellent UHF reports

The bad:
- I still can't get RX contact with our major VHF repeater in the area.
- I could not a squelch tail after transmission as I'm accustomed to hearing when on a rubber duck antenna.

 

[jwreich]

New update:

I continued testing by taking the comet and signal stick mag mounts on another vehicle to see if my truck is causing some issues.

Comet:
- I moved the comet to the center roof of a sedan. I was not able to get a squelch tail on the local VHF repeaters, but I was successful with the primary UHF repeater.

Signal Stick:
- The Signal stick heard a squelch tale on one local VHF repeaters, and the primary UHF repeater.

I tried to move the signal stick to the hood of my Dodge 2500. It was at this time I realized the 2020 Dodge Ram 2500 does not have a ferrous metal hood. I think it Aluminum.

It doesn't explain why the Comet mag mount had issues on an older all steel sedan, but I understand it does contribute to issues with antenna placement. I read where Ford resolved this with some bonding, but I doubt Dodge did anything.

Does any of this make sense?

 

[mmckenna]

Does any of this make sense?

Not really.
Aluminum is still a conductive metal. In fact, it has less resistance than steel. Sure, the magnet won't stick to it, but it's still a ground plane.

Mounting the antenna off to the side makes the antenna slightly directional. May be working against you in this case.

Don't ever rule out that there isn't an issue with the repeater itself.

 

[jwreich]

Thank you @mmckenna. I have verified the repeater is operational by switching my HT to a Diamond RH707 rubber duck antenna and making contact with a local ham.

All of my testing is with the engine(s) off so I don't induce any addition interference.

 

[mmckenna]

I'd ditch the Comet.

Get a known good antenna from a reputable manufacturer. Ham grade antennas are not known for their durability. Sounds like yours may have some internal damage. Antennas like this do not last forever.

 

[jwreich]

I'm happy to borrow an antenna from a local ham but both of these comets are less than 30 days old form DX Engineering. That's the crazy part.

I have reached out to our local Elmer group and asked for some help tonight. I hope to hear from someone soon.

 

[mmckenna]

I'm happy to borrow an antenna from a local ham but both of these comets are less than 30 days old form DX Engineering. That's the crazy part.

When I first got my ham ticket, I bought a ham radio oriented mobile antenna. It lasted a few weeks. Full size truck and a tall antenna met a tree and it didn't end well.
A neighbor who had experience in professional two way radio turned me on to Larsen antennas. I bought one and used it for about 2 decades.

All antennas are not created equal.

Mounting location matters.

There's a lot of deep science involved in antennas (some will even say a healthy dose of voodoo), so weird issues are challenging. It's very difficult to troubleshoot antennas over the internet. Likely someone with the right knowledge and test equipment can help you narrow this down.

I have reached out to our local Elmer group and asked for some help tonight. I hope to hear from someone soon.

Good luck. That's a good plan.

 

[tvengr]

Since the one radio works and the other doesn't on the same antenna, I believe it is an interference issue. The 2 radios probably use different IF frequencies. One may have the local oscillator operating above the incoming frequency and the other below. If you have any USB adapters in the vehicle, unplug them. Does the problem occur with the ignition off. If it doesn't, you probably have the vehicle computer radiating signals. You need to reroute the antenna coax away from the computer and computer cables. Check if the problem occurs with the headlights on and off. LED headlights cause a lot of problems. If that is the case, you need to install ferrite bead clips on the headlight cables.

 

[G7RUX]

Ok, I tried to read up on Smith Charts and all of the data included.

When I touch the NMO base of the antenna, not the physical mount, the impedance and SWR numbers jump all over the place. I am going to try and get some screen shots.

When I touch the physical mount in the truck, nothing changes.

When I touch the antenna above the NMO base, nothing really changes.

I did notice the the SWR jumps up when I move my hand from the connection jumper of the NANO VNA.

I also have the same antenna on a Comet Mag mount. It exhibits the exact same lack of RX characteristics when positioned in the middle of the roof. When testing it with the NANO VNA, it does not fluctuate in SWR regardless of my hand placement.

I'm going to put a call out to some elmers in the area for assistance.

Morning,

Apologies for not replying sooner but I have been busy playing trumpet...

So, Smith charts *look* complicated but they aren't really, especially if you are familiar with complex numbers. In short you would hope for a lovely stable dot on the 50 Ohm resistive point (usually the centre of the chart) but a good typical real antenna will usually give a curve or loop near the 50 Ohm point. It's worth getting to grips with them as they tell you a *lot* of information in the one drawing.

If the numbers are very variable when you touch the antenna base then I would suspect that the mount is not correctly installed, specifically around the bonding to the roof/ground plane. That the magmount doesn't do this when you touch it would suggest that the capacitive coupling to the roof is pretty good.

Diagnosing the issue remotely is difficult. However I would suggest that a local elmer would be a really good way to go since they can probably assist you with A/B testing with a ton of different bits and bobs and they could even do on-air testing direct to you.

That you report decent performance on UHF is telling...it would be expected that performance at UHF would be worse than at VHF if any difference were noted, just because losses, etc get worse as frequency increases. Capacitive effects get better as frequency increases, so it is possible to get UHF "jumping" a break better than one would see with VHF.

If you are able then I would suggest scanning around to see if there are any very strong signals knocking around (think pagers, etc) since these can cause all manner of difficulties in receivers (and transmitters in extreme cases.)

For testing the installation and antennas, I would go the following route...
1. Check DC continuity, as you seem to have already done.
2. Pop a 50 Ohm load on the antenna mount and puff the system through with the nanoVNA...You should see an excellent match but it is very unlikely to be 1:1. This should be very stable when touching the mount and load.
3. Without changing anything else, remove the load and connect the antenna. Check the VSWR with the nanoVNA which is still connected...this should be noticeably worse than just the load but *should* still be reasonably good at the frequencies of interest. The VNA will show you where any tuning peaks are and these should be at or close to the frequencies of operation. You *may* need to adjust the lengths of the parts of the antenna to get things working nicely for the frequencies of interest.

I do note that the Comet CA-2X4SR has a PL259 base so I presume you are using an NMO-SO239 adaptor of some sort...are you certain that this is working correctly? Some NMO adaptors have "iffy" sprung centre pins and can cause connection issues...these often look better at UHF as the capacitance results in lower capacitive reactance at higher frequencies.

Sorry for the long reply!

 

[k6cpo]

It sounds like you're doing all your testing in your own driveway. Have you tried driving around and attempting to hit the repeater from other locations?

 

[tvengr]

It sounds like you're doing all your testing in your own driveway. Have you tried driving around and attempting to hit the repeater from other locations?

Great idea! That would eliminate any interference from sources in the immediate area. If the interference continues, concentrate on possible causes in your vehicle.

 

[jwreich]

Lot's of comments. I appreciate them.

Yes all testing is done in my driveway, but I'm fortunate enough to live on a hill in a rural area. The testing is done with the truck turned off, but I will test again after removing the single USB charger.

I will also test tomorrow driving around to see if that performs differently. I also have a metal roof on my house to I plan to put the Mag mount on my roof and test with that as a ground plane.

I have been talking all weekend on my HT (Retevis RA89) and a Diamond RH707. Tonight (saturday) I was able to participate in a Simplex net with Net Control 5-6 miles away as the crow flies. There is a lot of organic matter and made acceptable contact on both VHF (146.55) and UHF (446.000).

More testing on Sunday. I appreciate all the feedback.