VHF not effective while UHF is

[wqzd805]

I needed a simple method to setup a reliable communication link between 4 HTs at a very remote hunting cabin.

It had to be easy to transport and simple to setup so a full duplex system with cavity filters, bandpass, ect.... was never a consideration.

A crossband repeater was however.

What I bought and put together is:
TYT-7800 crossband radio
Comet GP-3 base antenna
75' LMR400 UF

I added a 120mm fan on the heatsink of the TYT.

I temporarily set it up at my house to test coverage by mounting the antenna to a 10' wooden 1"x1" and place it in a tripod on top of my two story house.

We use UV5R at the hunting location (please dont turn this into a "baofeng is garbage" contest). The radios all have replaced nagoya NA-771 antennas.

While conducting a range test, the abililty of the radio or radios to transmit to the crossband on VHF/ receive UHF was extremely poor. Less then 1 mile. When I changed the VFO and worked the crossband in the other direction, transmitting UHF/ receive VHF, I was able to make communication out to 7 miles.

My house is in a particular awful location. Its in a low elevation between two dips in the terrain. The base antenna cant ever make it above the average tree line unless I get way up there - 100' roughly.

So considering that, I felt 7 miles was exceptional. I would imagine at the hunting cabin which isnt faced with this problem, communication will be even further.

Anyway. Why is it that VHF from the HTs to the CB repeater was so terrible? I even attached the HT to a SO239 mag mount with a comet ca-2x4sr and then switch handheld to a TYT 8000d 10w. Still nothing was making into the CB repeater over VHF.

Its confusing to me because transmission from the repeater over VHF was that much better then UHF, meanwhile on the HT side - it is the exact opposite.

Is this a common problem. My local LEO all use VHF handhelds but they obviously use a full duplex vhf/vhf system but the fact still remains.

Could something be wrong with the Comet GP-3? I did remove the center antenna element and inspected the capacitors and soldered contacts. It looked fine.

However, the ground radials dont seem correct? They are the length (about 6") to effectively work will on UHF but no so much on the VHF side of things.

Im going to go tinker with it now by attaching taping some steel wire to increase their length to 20" and see if that helps.

I just cant believe VHF transmission to the CB repeater was 3/4 mile at best. VHF is supposed to get through foliage and growth better.....whats up with this setup?

 

[K4EET]

wqzd805, firstly, welcome to Radio Reference...

If I may ask, what specific frequencies do you have programmed for use on VHF and UHF?

Dave

 

[popnokick]

Least likely to be the issue: The HTs, the TH-7800, or the GP-3 antenna. Most likely to be the coaxial cable / connectors. Will be very difficult to diagnose without some instruments. Specifically, you'll need at minimum an SWR meter capable of operating in the UHF band (as well as VHF, which most do) and if you can get one, an antenna analyzer with appropriate adapters for your coax and antenna connections. Since you posted this here in the Amateur Radio General Discussion forum I'll make the assumption that you know one or more hams that have the instrument(s). Otherwise, there are reasonably priced SWR meters and antenna analyzers available. You need some specific measurements to locate the issue.

 

[mmckenna]

Yeah, could be a number of things,

One drawback of VHF from a small portable radio is that the chassis of the hand held doesn't provide a very good ground plane at VHF frequencies.

Which frequencies you are using can come into play depending on what the antennas are tuned for. Since this is the "amateur" forum, we'll assume you are using 2 meters/70 centimeters.

Could be local noise sources.

Could be the transmitter desensitizing the receiver on the cross band repeat, depends on the filtering in the radio.


Could be something else, without details, it's hard to tell. Would be interesting to know more details on the set up.

 

[wqzd805]

Least likely to be the issue: The HTs, the TH-7800, or the GP-3 antenna. Most likely to be the coaxial cable / connectors. Will be very difficult to diagnose without some instruments. Specifically, you'll need at minimum an SWR meter capable of operating in the UHF band (as well as VHF, which most do) and if you can get one, an antenna analyzer with appropriate adapters for your coax and antenna connections. Since you posted this here in the Amateur Radio General Discussion forum I'll make the assumption that you know one or more hams that have the instrument(s). Otherwise, there are reasonably priced SWR meters and antenna analyzers available. You need some specific measurements to locate the issue.

I have a mfj845 swr. I’m using 467.550 input and 147.390 output with a dcs tone. The output on 2m is 1.1:1. Uhf is higher because I’m slightly outside of the antennas operating band, it reads 1.7:1 And I’m okay with that considering the antenna is tuned for 70cm.

Last night I did some more function test.
I setup the system the way it will be deployed when at the cabin and doing so I achieved much better results. More long the lines of what I was expecting.

Im using a tripod I altered to accept 20’ of aluminum mast, the GP3 is at the top.

The radio is in a ammo can powered from a Milwaukee M18 12aHr battery. I have a DC/DC constant current/voltage buck converter from the battery to the radio to being voltage down to 13.5v.

works great. I have a lot of Milwaukee batteries so it only make sense to use them to power the repeater. I sent a decent hour of traffic through the repeater last night and the battery dropped 7% in charge.

I was full quieting at 5 miles using vhf input.

7 miles was s5 and 10mile was s1 and drop off.

granted this was done from inside my vehicle.Getting out and transmitting gave much stronger audio reports But it was snowing and frigid cold last night so getting out past 10 miles wasn’t my priority.

range was already further then i need and at the cab in, the repeater antenna is going to be higher in elevation over terrain.

but back to my original problem, well i don’t know. Maybe the gp3 didn’t favor being attached to a wood mast, perhaps a metal mast improves upon the ground plain radials.

im not sure, maybe it was a null the way it was positioned on the roof. Even though it was about 35’ higher when on the roof it performed much much worse.

 

[AK9R]

I have a mfj845 swr. I’m using 467.550 input and 147.390 output with a dcs tone. The output on 2m is 1.1:1. Uhf is higher because I’m slightly outside of the antennas operating band, it reads 1.7:1 And I’m okay with that considering the antenna is tuned for 70cm.

Aside from your technical issues, you may have some legal issues.

467.550 MHz is a GMRS frequency. Do the stations transmitting on 467.550 MHz have GMRS licenses?

147.390 MHz is an amateur radio frequency. Do you have an amateur radio license? Even if you do, I don't think that cross-banding from GMRS to amateur radio is allowed by the amateur radio rules.

 

[mmckenna]

I have a mfj845 swr. I’m using 467.550 input and 147.390 output with a dcs tone. The output on 2m is 1.1:1. Uhf is higher because I’m slightly outside of the antennas operating band, it reads 1.7:1 And I’m okay with that considering the antenna is tuned for 70cm.

I take it you do understand the issues cross banding between amateur radio and GMRS, right?? If you all have your amateur licenses, move the GMRS channel to a 70cm frequency instead. Your base antenna will work better.

I suspect the metal pole helped the antenna work a bit better.

Good use of the Milwaukee batteries.

 

[wqzd805]

Aside from your technical issues, you may have some legal issues.

467.550 MHz is a GMRS frequency. Do the stations transmitting on 467.550 MHz have GMRS licenses?

147.390 MHz is an amateur radio frequency. Do you have an amateur radio license? Even if you do, I don't think that cross-banding from GMRS to amateur radio is allowed by the amateur radio rules.

I am aware but it isn’t common practice or routine. I have a ton at 70cm traffic in my area and I didn’t want to send countess comm checks over the air.

There is no gmrs, well there is but it’s on channel 21 -462.700. Plus, I was doing this during the late late hours of night as to not cause interference with other operators.

 

[jwt873]

You can't cross band between GMRS and Amateur frequencies.. If you haven't got your amateur license, that's even worse.

If you need to cross band between VHF and UHF, you might try going between GMRS and MURS.. Although I doubt even that's legal.

 

[k6cpo]

You can't cross band between GMRS and Amateur frequencies.. If you haven't got your amateur license, that's even worse.

If you need to cross band between VHF and UHF, you might try going between GMRS and MURS.. Although I doubt even that's legal.

I doubt it is. The only thing I've ever seen where it's legal to talk between radio services is the annual military<>amateur cross band test. And even this is set up in such a way that the amateurs transmit only on ham frequencies and listen on the military frequencies and vice versa.

 

[popnokick]

Your MFJ845 SWR meter covers 1.8 - 60 mHz and is useless for the frequencies in your setup.

 

[buddrousa]

History Lesson on a VHF Site for your money 7 miles is good.
#1 VHF Repeater $1000
#2 Good VHF Antenna $1000
#3 Good VHF Duplexer $1000
#4 Hardline $5 a foot 300 feet $1500
#5 300 foot tower $15000
= 1 VHF Ham Site $20000

 

[mmckenna]

I am aware but it isn’t common practice or routine. I have a ton at 70cm traffic in my area and I didn’t want to send countess comm checks over the air.

There is no gmrs, well there is but it’s on channel 21 -462.700. Plus, I was doing this during the late late hours of night as to not cause interference with other operators.

I'd really think that someone who has both a GMRS license and an Amateur Extra license would know better. But, whatever. I'm just not surprised by this sort of stuff anymore.

 

[bill4long]

If you need to cross band between VHF and UHF, you might try going between GMRS and MURS.. Although I doubt even that's legal.

Not legal for either service. Nor is it legal for amateur radio to retransmit from other services.

 

[W5lz]

Honest question... Why use a cross band system like that? I don't see a benefit.

 

[PrivatelyJeff]

Honest question... Why use a cross band system like that? I don't see a benefit.

Easy to setup and gets you better range versus pure handhelds. All you need is a radio, antenna and power.

 

[StaticDischarge]

Honest question... Why use a cross band system like that? I don't see a benefit.

Way less expensive equipment... I have two TM-V71's and two IC-W32A's and all four crossband repeat.. Can set up a heck of a system without much more than power supplies and dual band antennas and away we go.....

 

[W5lz]

So, you have a "system". Now what are you going to use it for that you couldn't do with out the cross banding? Using four radios for cross banding at the same time means tying up eight frequencies. Without some legitimate reason it's just a waste of bandwidth. So, what's the purpose? Or... is it just having 4 -opportunities- to do cross banding?

 

[KK4JUG]

I think the "Beofeng is garbage" concept is the least of your problems.

 

[Project25_MASTR]

Let's look at this problem from the view of a radio tech...

1. Does the "repeater" spec out correctly on the frequencies of interest in receive, transmit and full-duplex?
2. Is the antenna system known to be within the operating limitations of the system?
3. Does the portable spec out correctly on all frequencies of interest.

Unfortunately, to adequately answer those questions (which will tell you where the likely culprit is) one needs some knowledge and equipment to test it with. Also one of the reasons why I used to bill $200 per hour to answer those questions...