VHF/UHF radio and antenna questions

[AK9R]

I have some runs of DX Engineering's 400 Max and it works well for me in VHF/UHF applications.

 

[mmckenna]

Thanks, very good points you're making. Sounds like the old antenna pole would be a safer option, but may require more coax cable as its mounting brackets are on the back of the house. There's no worries about power lines, other buildings, vehicles, etc because of its location.

Being here ten years, lightning isn't as intense as it was in Texas where I used to live, or like Florida, which is probably the worst. But we do get occasional intense storms. We've not had any nearby strikes, it within 100 yards that I can think of. But that doesn't mean it can't never happen.

Yup. Even 100 yards might be too close.
Either way, the National Electric Code requires it, and it's a good idea. A lot of hams/hobbyists don't do it and survive. I've been to FL and TX quite a bit, but still the most violent storm I was ever personally in was in your part of KY.

So I guess I'd need some kind of inline lightning suppressor to keep such strikes from damaging any radio equipment? But also, should the metal pole have some kind of lightning dissipation means? That is, maybe some kind of connection at the bottom of the pole to a grounding rod?

It gets complex, unfortunately, but not impossible.
You'd want a ground rod at the base of the mast. You'd want the mast attached to that rod with a ground conductor and a clamp. You'd want your lightning protection device attached to that same rod with another clamp.
The NEC also requires that the new rod be bonded to the existing home rod. If your mast is right where the existing rod is, then you can use that, you just can't double up conductors on a clamp unless specifically designed for it.

Important that you bond the new ground rod and the existing ground rod together. You absolutely want that to prevent difference in ground potential.

The LMR400 cabling I'm looking at is from DX Engineering. It comes in various lengths, mostly PL-259 connectors. It sounds like I'd need some kind of adapter you're suggesting between the LMR and radio antenna input.

I agree with above, the DX engineering stuff seems to be reputable.

Yes, either order the cable with the correct connectors, or use an adapter, but use a short length of a smaller diameter flexible cable to make that final connection to the radio. That'll save you some headaches down the road.

So regarding the antenna's radiation pattern, are you saying that the longer the antenna, the more depressed the radiation pattern? So maybe the longest antenna wouldn't be the best for my location. Perhaps a middle length one would have a more elevated pattern?

Your mountains there are not like our mountains out here in the west. You'd be fine with a high gain antenna. My point was that it may not be needed. Since VHF/UHF tend to be line of sight, reaching repeaters usually doesn't require high power and high gain antennas. A medium sized antenna may work just fine, and be easier to install.

But the hobby is about experimenting and learning, so try something, you can always change it later.

 

[KY_Ham_64]

Hope everyone had a good Christmas. A quick question. I'm about to order my dual band antenna, probably be a Diamond X200A. I was thinking of hooking my HT up to it via a SMA to SO-239 adapter cord, about 3ft. I wanted to see how well it would pick up and transmit on the large base antenna before I put it up on my 19ft pole. Given that the SWR is say, no more than 2:1, that shouldn't be an issue for the HT should it? As in overload it or any other detrimental effect?

 

[mmckenna]

Given that the SWR is say, no more than 2:1, that shouldn't be an issue for the HT should it? As in overload it or any other detrimental effect?

How do you know the SWR is going to be that high? For the amateur bands, the antenna should be tuned so it has fairly low reflected power in the ham bands.

It's not going to hurt the HT. HT antennas are lousy at best and usually not something that provides a low SWR. Worst case is that the radio will warm up a bit more than usual. It's not going to damage anything if the SWR is a bit high.

 

[KY_Ham_64]

How do you know the SWR is going to be that high? For the amateur bands, the antenna should be tuned so it has fairly low reflected power in the ham bands.

It's not going to hurt the HT. HT antennas are lousy at best and usually not something that provides a low SWR. Worst case is that the radio will warm up a bit more than usual. It's not going to damage anything if the SWR is a bit high.

Ok thanks. I was just throwing the 2:1 out there as a worst case situation. The antenna's rated SWR is pretty close to 1:1-1.3 in the 70cm range, but has a bit of a steep curve on the 2m, but is supposed to be no higher than 1.7 between 144-148MHz.

 

[mmckenna]

Ok thanks. I was just throwing the 2:1 out there as a worst case situation. The antenna's rated SWR is pretty close to 1:1-1.3 in the 70cm range, but has a bit of a steep curve on the 2m, but is supposed to be no higher than 1.7 between 144-148MHz.

OK, got it.

That won't be an issue. Hand held radio antennas usually vary quite a bit on the SWR side since it depends on how the radio is held, where it is, etc. They'll tolerate a LOT of mismatch without damage.

 

[KY_Ham_64]

OK, got it.

That won't be an issue. Hand held radio antennas usually vary quite a bit on the SWR side since it depends on how the radio is held, where it is, etc. They'll tolerate a LOT of mismatch without damage.

I ended up ordering a dual band (2m/70cm) base and mobile antenna. For home, I got the Diamond X200A, and for the mobile, I got the Comet CA-2X4SR with a mag mount. Also picked up a VHF/UHF SWR meter.

I didn't get any coax yet as I'm concentrating on the mobile set up for now. I did get some short adapter cabling to run between the HT and antennas tho. Need to get the coax and grounding items next, and then put the X200 on my 19ft pole, but can't buy everything at once.

Just these few items cost a few hundred dollars. Further into the money pit I go..but you gotta pay to play.

Question about the 2x4SR antenna, do they have pretty decent gain? I know it won't be like X200, but should be quite a bit better than the HT antennas. The datasheet claims 3.8/6.2dBi gain. Does the metal roof/hood/trunk compose the ground plane, kinda like radials do for a vertical ground antenna?

 

[mmckenna]

Question about the 2x4SR antenna, do they have pretty decent gain? I know it won't be like X200, but should be quite a bit better than the HT antennas. The datasheet claims 3.8/6.2dBi gain.

That's pretty high gain for a mobile antenna. It will be better than the HT antenna, especially if you were trying to use the HT antenna while it was inside the car. Getting an antenna, nearly any antenna, outside the car will result in a big improvement.

Does the metal roof/hood/trunk compose the ground plane, kinda like radials do for a vertical ground antenna?

Yes.

 

[prcguy]

I ended up ordering a dual band (2m/70cm) base and mobile antenna. For home, I got the Diamond X200A, and for the mobile, I got the Comet CA-2X4SR with a mag mount. Also picked up a VHF/UHF SWR meter.

I didn't get any coax yet as I'm concentrating on the mobile set up for now. I did get some short adapter cabling to run between the HT and antennas tho. Need to get the coax and grounding items next, and then put the X200 on my 19ft pole, but can't buy everything at once.

Just these few items cost a few hundred dollars. Further into the money pit I go..but you gotta pay to play.

Question about the 2x4SR antenna, do they have pretty decent gain? I know it won't be like X200, but should be quite a bit better than the HT antennas. The datasheet claims 3.8/6.2dBi gain. Does the metal roof/hood/trunk compose the ground plane, kinda like radials do for a vertical ground antenna?

The 2X4SR is about the highest gain wide band dual band mobile antenna out there for amateur through GMRS and MURS. It should be about 2-3dB higher than a 1/4 wave whip on UHF and maybe like a 5/8 on VHF.

 

[kayn1n32008]

Ok thanks. What would you say the gain of your X510 would be? Is there a way to measure/calculate it?

Keep in mind 'gain' is achieved by squishing the vertical radiation pattern and forcing the energy out to the horizon. When in mountains, gain can work against you.

 

[KY_Ham_64]

The 2X4SR is about the highest gain wide band dual band mobile antenna out there for amateur through GMRS and MURS. It should be about 2-3dB higher than a 1/4 wave whip on UHF and maybe like a 5/8 on VHF.

If it can work on MURS and GMRS, and the others, wonder if it'd work on 1.25m?

If the metal of the car is the ground plane, then I could use like a large pizza pan as one, provided it's like 3.5ft (1/2 wavelength) in diameter or so to make it work for 2m?

 

[prcguy]

If it can work on MURS and GMRS, and the others, wonder if it'd work on 1.25m?

If the metal of the car is the ground plane, then I could use like a large pizza pan as one, provided it's like 3.5ft (1/2 wavelength) in diameter or so to make it work for 2m?

Its not advertised as working there and if it could you can bet they would be telling you about it.

 

[scanmanmi]

I could get a few miles with a 1/4 wave at 10'. I went to 30' with LMR400 and a Comet GP9 and I can get half the state of Michigan now. I'm a little elevated but where there are no real hills I can talk to my mobile at 40 miles.
Go to Le site web de Radio Mobile and create a free account (it may appear in French). Click Radio Mobile Online. You can enter your antenna and radio info and it will show a coverage map.
If you go to Google Earth click Add, Path, click your location, zoom in or out, click on other location, title it and hit OK. Now that your path is there, right click on the path and go to Show Elevation Profile. This will show you if you need any antenna height to get over the hills.

 

[KY_Ham_64]

I could get a few miles with a 1/4 wave at 10'. I went to 30' with LMR400 and a Comet GP9 and I can get half the state of Michigan now. I'm a little elevated but where there are no real hills I can talk to my mobile at 40 miles.
Go to Le site web de Radio Mobile and create a free account (it may appear in French). Click Radio Mobile Online. You can enter your antenna and radio info and it will show a coverage map.
If you go to Google Earth click Add, Path, click your location, zoom in or out, click on other location, title it and hit OK. Now that your path is there, right click on the path and go to Show Elevation Profile. This will show you if you need any antenna height to get over the hills.

Thanks I appreciate the info, I'll try it oot.

Got part of my order in yesterday, the mag mount, swr meter, various cables and connectors. But no antennas, they're supposed to be here later in the week.

The mag mount has a white film on the bottom of it, and has the rubber ring around the outer edge. I'm guessing the white film stays on it to protect the car surface from the magnet, but still makes for a good magnetic attraction to the metal?

 

[mmckenna]

The mag mount has a white film on the bottom of it, and has the rubber ring around the outer edge. I'm guessing the white film stays on it to protect the car surface from the magnet, but still makes for a good magnetic attraction to the metal?

Yes, leave the film on there. The rubber boot is to reduce damage to the paint.

 

[KY_Ham_64]

Well my antennas finally got here, guess we were the last of the UPS delivery list for the day. I got out the CA2X4 mobile antenna and mounted it to the mag mount and took it outside and plunked it on the roof of my car. Reception seems very good. I actually was able to pick a net meeting on a repeater 35 miles to the east. Very impressed. But I wasn't able to join in with my little 5W HT. I was out there in 23 degree cold trying out my new toys..

I hooked up the SWR meter between the HT and antenna and noticed with a bad ground plane the VHF SWR was around 3:1, but when I put it on a magnetic metallic surface that dropped down to about 1.2-1.5. UHF didn't seem to make a difference what the ground plane was.

Oh well, time for the experiments to begin..got to get the big X200 out and see what it's capable of. Maybe tomorrow when it'll be above freezing..

 

[KY_Ham_64]

I took my SWR meter outside today to test various locations, but mostly on my car (2003 Honda Accord). I plunked the mag mount and CA2X4 on the center of the roof just behind the moonroof. To my surprise on VHF, the readings were about 2:1. On the hood it dropped to about 1.7, on the trunk about 1.8. The best readings were about 1.4-1.5 on the center of the roof close to the rear window.

Is this the reason you see a lot of antennas on the rear of the roofs? Why is this? Is it because it's closer to the actual metal frame? Does this location offer a more efficient ground plane or radiation pattern?

 

[mmckenna]

I took my SWR meter outside today to test various locations, but mostly on my car (2003 Honda Accord). I plunked the mag mount and CA2X4 on the center of the roof just behind the moonroof. To my surprise on VHF, the readings were about 2:1. On the hood it dropped to about 1.7, on the trunk about 1.8. The best readings were about 1.4-1.5 on the center of the roof close to the rear window.

Is this the reason you see a lot of antennas on the rear of the roofs? Why is this? Is it because it's closer to the actual metal frame? Does this location offer a more efficient ground plane or radiation pattern?

The antenna wants to see a quarter wavelength of ground plane in all directions directly under the antenna.
I suspect your moon roof is resulting in a less than ideal ground plane (they usually do). Hood is a big ground plane and should be OK. Trunk is usually smaller and may be an issue. Also, it's below the roof of the car, so you'll often get some amount of reflection off the roof/vertical sides.

When I install antennas, my goal is to make sure there is a 1/4 wave of groundplane under the antenna base. That's easy to do on VHF and higher frequencies on a full size truck/suv, etc. Not hard to do on a car roof, if it doesn't have the moon roof on it.

The ground plane wants to be -directly- under the mount.

Sounds like for your specific application and with the moon roof, towards the back of the roof is working out best.

But, keep in mind, mag mounts come with some drawbacks. They space the antenna a bit above the ground plane. They don't have a direct connection to ground like a permanent mounted antenna would. The way the coax is run can impact things.
Also, proximity to humans, doors open, metal buildings, lamp posts, etc. can all impact it.

1.4:1 isn't bad. I'd not mess with the antenna if that's what it's giving you on that mount.

 

[KY_Ham_64]

The antenna wants to see a quarter wavelength of ground plane in all directions directly under the antenna.
I suspect your moon roof is resulting in a less than ideal ground plane (they usually do). Hood is a big ground plane and should be OK. Trunk is usually smaller and may be an issue. Also, it's below the roof of the car, so you'll often get some amount of reflection off the roof/vertical sides.

When I install antennas, my goal is to make sure there is a 1/4 wave of groundplane under the antenna base. That's easy to do on VHF and higher frequencies on a full size truck/suv, etc. Not hard to do on a car roof, if it doesn't have the moon roof on it.

The ground plane wants to be -directly- under the mount.

Sounds like for your specific application and with the moon roof, towards the back of the roof is working out best.

But, keep in mind, mag mounts come with some drawbacks. They space the antenna a bit above the ground plane. They don't have a direct connection to ground like a permanent mounted antenna would. The way the coax is run can impact things.
Also, proximity to humans, doors open, metal buildings, lamp posts, etc. can all impact it.

1.4:1 isn't bad. I'd not mess with the antenna if that's what it's giving you on that mount.

Thanks. Of course the hood is not an option, but I guess it's reading lower SWR because it's a large ground plane. The farther back on the roof the better, but that's where it starts sloping a bit and the mag mount doesn't sit as flush in it as it does a foot further forward.

Will having the antenna on the roof make it immune from RF interference from the car's electrical system while it's running?

I noticed when I TX on UHF it's only putting out about 2-3W. On VHF it's 5W but not on UHF. So I don't know what's going on there. On this HT the power settings are 0.5, 2.5, and 5W, and I have everything set to 5W.

 

[mmckenna]

Thanks. Of course the hood is not an option, but I guess it's reading lower SWR because it's a large ground plane.

I had a co-worker with a jeep and he was an avid ham operator. He needed a suitable ground plane for his VHF antenna. He understood the importance of the ground plane and how that improved performance when way out in the middle of nowhere. He had a 5/8'ths wave VHF whip dead center in the hood of his jeep. Function before form.

The farther back on the roof the better, but that's where it starts sloping a bit and the mag mount doesn't sit as flush in it as it does a foot further forward.

In your specific application, that seems to be what is helping. But if you didn't have the moon roof, dead center of the roof would give you the best performance.

Remember, low SWR isn't the measure of a good antenna, it just means that something between the SWR meter and the ether is sucking up all that RF and it's not getting reflected. A 50Ω dummy load will give you perfect SWR, but won't radiate a signal worth a can of beans.

Radiation pattern is important. Getting the antenna on a surface with a suitably sized and equal ground plane in all directions will give you a true omni-directional radiation pattern. Any offset that results in less than 1/4 wave of ground plane under the antenna will result in a lopsided ground plane.

Will having the antenna on the roof make it immune from RF interference from the car's electrical system while it's running?

No, not immune, but distance from the noise source helps. Usually the roof is about as far away as you can get, and puts that big ground plane between the antenna and the noise source. RFI can still get in via poor power wiring.

I noticed when I TX on UHF it's only putting out about 2-3W. On VHF it's 5W but not on UHF. So I don't know what's going on there. On this HT the power settings are 0.5, 2.5, and 5W, and I have everything set to 5W.

Unless you are transmitting directly into a perfect 50Ω load, the power meter readings should not be trusted.