Power required for max DX with a 1/4 wave antenna on VHF?

[Zakowsky2]

OK, so the 1/4 wave ground plane vs 1/2 wave NGP vs 5/8 wave GP discussion is very well documented and diagrammed online. In short, the 1/4 wave has a rounded pattern that sends more signal vertically, whereas the 1/2 and 5/8 wave have flatter, further reaching horizontal lobes. So the 1/2 wave is better for flat landscapes because you can get more distance per watt, but if you are in hilly terrain, a 1/4 wave could be an advantage to get over the landscape.

After my 1/2 wave was ripped off my truck, before buying a new one I put my old 1/4 wave mag mount back on and for interest ran some tests. I have a route I go where there are low “canyon” type areas that transmitting out of is usually more difficult. I found the 1/4 wave was in fact almost twice as good at getting out of the canyon as the much taller 1/2 wave, so much better that I was quite surprised.

So a 1/4 wave doesn’t have as far a range per watt on a flat landscape – but with a maximum range of 160 kms due to the curvature of the earth, can’t this just be overcome with power? If you had enough power, wouldn’t the 1/4 wave be the best solution by far, as the vertical component would handle the hills better, and the ~20% loss in horizontal range (compared to a 1/2 or 5/8 wave) easily overcome with a few more watts?

So to the question finally – how much power do you need to push a 1/4 wave antenna out to 160 kms on VHF, if mounted on the roof of a truck? I have 80 watts in my mobile and although I didn’t have much time to test, I made it easily to 130 km repeaters on the flats.

Btw this all part of trying to decide if I should stick with my roof rack mounted 1/2 wave (because of the not so great GP of a roof rack mount) or to mount a 1/4 wave directly to the roof (which has problems too because I can’t put it in the center). Or put the 1/4 wave on the roof rack without such a good GP…so many combinations.

Thanks

 

[AK9R]

A 5/8 wave vertical whip antenna is generally accepted to have 3 dB of gain relative to a dipole while the 1/4 is generally accepted to have 0 dB gain relative to a dipole.

So, using a 1/4 wave antenna instead of a 5/8 wave antenna means that you need to make up for 3 dB of gain.

3 dB of gain is equivalent to a doubling of output power (actually a doubling of power is 3.01 dB of gain). If you are using a 50 watt radio with your 5/8 wave antenna, you'd probably need 100 watts into a 1/4 wave antenna to have the same "range". Of course, this is all theoretical.

I think the Larsen catalog quotes 2 or 2.5 dB of gain for their 1/2 wave antennas.

 

[Zakowsky2]

Of course, thanks AK9R - I guess I read so many times that the gain numbers are not accurate, but for comparative purposes, they answer the question. Next time I have time I am going to see if my 1/4 wave can make 160 km with 80 watts.

It is also dawning on me why the J pole design works well, combining the wavelengths. I guess what I want is to mount a 1/4 wave right next to a 5/8 on the roof!

Thanks again.

 

[mmckenna]

I think the Larsen catalog quotes 2 or 2.5 dB of gain for their 1/2 wave antennas.

Yes, but that's when mounted on an appropriate ground plane.

Mounting a 1/2 wave antenna on a luggage rack does not provide the appropriate ground plane, so those dB numbers drop to essentially zero, same as the 1/4 wave.


At OP:

The difference in mounting is part of the equation here. If you are concerned about maximum range, make sure you are doing everything you can to give your antenna what it needs. Drill the hole, install a permanent mount base that has proper contact with the body metal, smack in the middle of the roof. Anything else is a compromise.

I've done a fair amount of comparisons between antennas (unscientifically), and I've always gone back to the 1/4 wave antennas as the best all around performer for my specific applications.

There's also a lot more involved than just cranking up the power. Curvature of the earth is one issue. Local terrain, terrain near the receiver, antenna height, receiver sensitivity, receiver antenna, all play into this. General rule with amateur radio is to use the least amount of power necessary to establish communications.


In general:
New hams, actually many seasoned hams too, seem to underestimate the importance of proper antenna installation and a good ground plane. I see a LOT of hams with funky antenna installs that are severely reducing performance.
It's also a common new ham thing to assume that more RF power is the solution to every problem. With a good repeater, they'll have a high gain antenna designed for the coverage area (or at least "should"). They'll also usually have better receivers that will pull weaker signals out from the fringes of coverage. Many repeaters are designed for hand held radio coverage over some amount of area, and mobile beyond that. Often blasting more power at a repeater isn't necessary.

 

[Zakowsky2]

Thanks McKenna; yes, I am in fact trying to learn/sort out my ground plane more than anything, and that is dictating the wavelength of the antenna more than the other way around in my case. I don’t have a problem drilling the hole, it’s just when my truck is set up for Overlanding there is no way to drill it in the center of the roof, I am committed to the gear that gets mounted there. So I was trying to figure out which would be better, a hole in the roof that is way off to the side or a corner, then use a 1/4 or 5/8, depending on if I am in flat or hilly terrain, or use the rail mount (so no good GP) with a 1/2 wave which I thought doesn’t need a ground plane.

This is not to just hit repeaters around town, the goal is when I am up in northern British Columbia or Alaska way it gets pretty remote, and then there is the mountains. So it is nice to hit a repeater when you can find one, but it can be simplex to other trucks as well.

So yeah, roof rack mount with a 1/2 wave vs an off-center hole on a big steel roof, with a 1/4 wave or 5/8, depending on the landscape. I also am thinking about a 1/4 wave NMO to the rack mount, but use grounding straps to the metal of the roof itself. Those are my 3 options I am trying to decide between.

 

[AK9R]

Absolutely, a proper ground plane is required in order to get the gain figures that Larsen, Laird, MaxRad, etc., advertise. Anything other than the center of the roof will potentially not meet spec and will likely show some directionality.

As for J-poles, there's no magic. They are an end-fed halfwave antenna. That's the upper section with one element. Because a halfwave antenna has a high impedance at the end, it needs something, such as a transformer, to match the antenna's impedance to the coax cable impedance. The lower section of the J-pole with the dual element. That's the transformer. Which is why you need to keep the lower section of a J-pole away from anything metallic that might upset the magnetic fields around the transformer.

 

[mmckenna]

So I was trying to figure out which would be better, a hole in the roof that is way off to the side or a corner,

As long as there is a 1/4 wavelength at your lowest frequency around the base of the antenna, you are good on the ground plane. Does not matter if it's a 1/4 wave, 1/2 wave or 5/8th's wave antenna, a quarter wave ground plane is what you want. So, for 2 meter band, you want about 19" in from any roof edge. Anything less and you'll get some directional radiation off the antenna.

Center of the roof is ideal, but in your case, that's not going to work. You do want to figure out how to deal with interactions with the roof rack if it's close to the antenna. That'll create some issues.

then use a 1/4 or 5/8, depending on if I am in flat or hilly terrain, or use the rail mount (so no good GP) with a 1/2 wave which I thought doesn’t need a ground plane.

1/2 wave antennas do not -require- a ground plane, but they work better with one:
A 1/2 wave with no ground plane is 0dB gain.
A 1/2 wave with a ground plane is around 2.1dB gain. So, benefit to giving it a proper ground plane, as it impacts the radiation pattern of the antenna.

Swapping antennas is a good option. Use what works best for your environment. That's the beauty of the NMO mount, you have a wide selection of antennas to choose from.

This is not to just hit repeaters around town, the goal is when I am up in northern British Columbia or Alaska way it gets pretty remote, and then there is the mountains. So it is nice to hit a repeater when you can find one, but it can be simplex to other trucks as well.

So yeah, roof rack mount with a 1/2 wave vs an off-center hole on a big steel roof, with a 1/4 wave or 5/8, depending on the landscape. I also am thinking about a 1/4 wave NMO to the rack mount, but use grounding straps to the metal of the roof itself. Those are my 3 options I am trying to decide between.

Ground straps/DC ground isn't the same as a ground plane. Like I said above, you want one quarter wavelength in all directions around the base of the antenna. If you can do that with your roof rack (mount a plate, or other conductive surface) you can absolutely do that.

I see a lot of overlanders that run into this challenge. Antenna mounting becomes a challenge with all the stuff up on the rack. Prioritizing your needs is important. Radio can be useful, but you need your camping gear, too. Sometimes a compromise is in order.

Hopefully you've got other means of communications in an emergency.

 

[Zakowsky2]

Great info both of you guys, thanks. I am going to get out there with a measuring tape and see if I can get 19”. The 1/4 wave I am testing with is a mag mount, so I can try different positions and see. Not as good as a hole I know but it should be relatively similar when comparing different spots. Will try it out and report back.

For other means of communications, I also bring an HF radio and a Hustler 40m hamstick which I put up when stopped (for fun). I fool myself that it would be useful in emergencies (I can hit California from Alberta with it), but I can’t fool my wife ! So for serious trips, a satellite backup would be rented.

 

[mmckenna]

Great info both of you guys, thanks. I am going to get out there with a measuring tape and see if I can get 19”. The 1/4 wave I am testing with is a mag mount, so I can try different positions and see. Not as good as a hole I know but it should be relatively similar when comparing different spots. Will try it out and report back.

Yeah, the mag mount is a good tool for testing. There's a slight improvement for the permanent hole, plus it reduces the chance of damage to the coax cable, stray RF, etc.

For other means of communications, I also bring an HF radio and a Hustler 40m hamstick which I put up when stopped (for fun). I fool myself that it would be useful in emergencies (I can hit California from Alberta with it), but I can’t fool my wife ! So for serious trips, a satellite backup would be rented.

I agree with your wife on HF. A cool tool, but no way I'd rely on amateurs in an emergency.

I carry a Garmin InReach Mini with me when I'm out away from everyone else. Never needed it for an emergency, but it's a very handy tool for checking in periodically. I pay $12 USD a month for my service.

 

[Zakowsky2]

Working from home is such a bad idea…

So I couldn’t wait, and went out and tested the roof position with the 1/4 wave mag mount. Interesting. On the left is my APRS track, 30 second intervals, 80 watts, with the antenna in the center of the roof. There is a sunroof, but there is still at least 19” of steel all around it (Landcruiser). On the map “flat” is where the landscape opens up and you can see directly all the way to the mountains to the west, about 70 km away. I used to live in California and “canyon” is not really a canyon, the road is only about 100-200 feet down but the walls are steep in places, and there are power lines that run along that side, and traditionally it is hard to get signal along that side. So the center mount was much better at getting out of there, just going roughly by how many checkpoints were missed – the curvy parts are the deepest, and the corner mount completely failed in some of them. But on the flat main road the corner mount was better, suggesting maybe some directionality as was mentioned perhaps. And given most of the repeaters are east of this map, and the antenna was on the west corner of the vehicle while going north through the canyon, maybe what this comes down to is just directionality? Total number of hits was about the same.



So this still leaves me stuck for what to do. I forgot I guess that 19" is almost half of the 42" roof, so a side mount is my only option. Maybe my original plan to use a 1/2 wave (so less dependent on a GP) on the side is better. I did this same test with it a long time ago, I am going to see if I can find the track on APRS.

 

[Zakowsky2]

Update, with what is maybe some useful info. I found my old logs on APRS and did have that route with my (now gone) 1/2 wave mounted on the roof rack rails (Diamond NR770HBNMO on a K515SNMO mount). Red dots are successful APRS hits (30 sec interval). Take home I think is they are all close, but 1/4 wave is better in the valleys, 1/2 wave better on the flats, as expected. But relevant is that the 1/2 wave with no good ground plane is in fact performing as well as the 1/4 wave roof mount in the center.


I guess the 1/2 wave (or 5/8) in the center of the roof would be best, but that is not an option. So I think I will fix my roof rack mount and buy another 1/2 wave NMO, and keep the mag mount 1/4 in the truck in case I have problems in valleys. Next test after I get the new setup will be to run a grounding strap from the mount to the roof, see if that makes any difference. Thanks for all the input.

 

[mmckenna]

But relevant is that the 1/2 wave with no good ground plane is in fact performing as well as the 1/4 wave roof mount in the center.

Both should have 0dB gain, so it's expected they'd be similar.

So I think I will fix my roof rack mount and buy another 1/2 wave NMO, and keep the mag mount 1/4 in the truck in case I have problems in valleys. Next test after I get the new setup will be to run a grounding strap from the mount to the roof, see if that makes any difference. Thanks for all the input.

With your setup, the 1/2 wave on the rack is probably what's going to work given all the other factors involved. I'd find a way to permanently mount it to the rack. They do make 90º NMO mount brackets that you could screw into the rack.

Ground straps may or may not make a difference, as DC ground and RF ground plane are different things.

 

[Zakowsky2]

Both should have 0dB gain, so it's expected they'd be similar.

Do I have this wrong? I thought the 1/4 direct mounted (or mag mounted to a degree) in the center of the roof itself would use the roof as a ground plane, and therefore get the 2 dBi gain? So it would be 1/4 with 2 dBi gain = 1/2 wave with 0dB gain. No?

Edit - to add, for the RF ground I was going to try and run as short a braided cable as I can from where the NMO part bolts into the rack mount bracket, directly down to the roof itself. The bracket is isolated from the rack by a sheet of rubber protecting the rail anyway. But thinking about it, if I can find an NMO surface mount with a swivel joint (the roof isn't flat in that area), and put a 1/2 wave on that, maybe that would be the best for me.

 

[prcguy]

Do I have this wrong? I thought the 1/4 direct mounted (or mag mounted to a degree) in the center of the roof itself would use the roof as a ground plane, and therefore get the 2 dBi gain? So it would be 1/4 with 2 dBi gain = 1/2 wave with 0dB gain. No?

Edit - to add, for the RF ground I was going to try and run as short a braided cable as I can from where the NMO part bolts into the rack mount bracket, directly down to the roof itself. The bracket is isolated from the rack by a sheet of rubber protecting the rail anyway. But thinking about it, if I can find an NMO surface mount with a swivel joint (the roof isn't flat in that area), and put a 1/2 wave on that, maybe that would be the best for me.

A 1/4 wave ground plane has less gain than a 1/2 wave dipole because its missing half the antenna. The ground plane doesn't radiate and the aperture size is smaller. Depending on who you talk to a 1/4 wave ground plane can be 1 to 3dB less gain than a 1/2 wave dipole in free space. A 1/2 wave dipole has 0dBd gain or 2.14dBi gain so subtract something from that for a 1/4 wave ground plane. Mounted above ground will change things and so will metal masts, etc.

 

[mmckenna]

Do I have this wrong? I thought the 1/4 direct mounted (or mag mounted to a degree) in the center of the roof itself would use the roof as a ground plane, and therefore get the 2 dBi gain? So it would be 1/4 with 2 dBi gain = 1/2 wave with 0dB gain. No?

Yeah, what prcguy said. The 1/4 wave won't have 2dBi gain.

Edit - to add, for the RF ground I was going to try and run as short a braided cable as I can from where the NMO part bolts into the rack mount bracket, directly down to the roof itself. The bracket is isolated from the rack by a sheet of rubber protecting the rail anyway. But thinking about it, if I can find an NMO surface mount with a swivel joint (the roof isn't flat in that area), and put a 1/2 wave on that, maybe that would be the best for me.

I don't think that's worth the effort, and I don't think it's worth drilling a hole in the roof to make a proper connection. DC ground and RF ground planes are different things. You have a bit of a ground plane with your rack. DC grounding it with a strap probably isn't going to make much of a difference, especially since the roof will be below the rack. If you want to DC ground it (and that's not a bad thing), do it somewhere that you don't have to drill into the roof. If this is a pickup, do it through the bed stake pocket. If it's an SUV, do it to an existing bolt somewhere that's easy to get to.

There's a lot of different mounts you can use, but keep it simple. Just a bracket with a 3/4" hole is fine. The ham radio oriented fold over mounts are going to hold the antenna base above the rack/roof even further, which won't help you. Keep it simple.

 

[K6GBW]

Don't let perfect be the enemy of good enough. A quarter wave mounted on a good NMO mount, even if it's closer to the edge than ideal, will still work relatively well. For overlanding the quarter wave is a great antenna. It's small and doesn't hit trees as much and it'd cheap so you can carry a spare. Typical simplex distances when overlanding should be fine with a quarter wave and anything from 25 to 50 watts. I've had VHF quarter waves mounted less then 12 inches from the edge of a roof and they worked fine. Yes, they are a little directional, but it's not a dramatic thing. On my pickup trucks I've typically mounted them toward the back of the cab, so they are directional toward the front. Also, don't get so caught up in gain. In the real world, under typical real use conditions, the quarter wave provides plenty of distance. It's also better if you have any elevation changes, like when talking into a repeater.

My current car is a Subaru Crosstrek and it has a UHF low profile antenna mounted on an NMO on the driver's side rear. Yes, it would have been better in the center of the roof, but like you, I carry stuff there sometimes. On balance, it's been a great set up and meets my needs.

 

[alcahuete]

I'm not sure why nobody has said it (or if they did and I missed it) but 160km (100 miles, for us yanks) on a 2m mobile setup is not going to happen reliably regardless of which antenna you choose or how much power you use. Just not going to happen.

Some high level repeaters are going to give you that kind of range, and every now and then you might be able to do it on completely flat terrain with no obstructions, but to attempt to get that kind of range with any sort of reliability, especially in mountainous terrain, or simplex, is an absolutely futile effort.

 

[Zakowsky2]

First off thanks for the last few posts, great information for Overlanders and truck mounting! I am going to start ordering some parts and see what I come up with.

Regarding the 100-mile limit, I think the key is “reliably”. It may be just because I am geek, but I like seeing how far I can reach! I guess I am the same on HF – when no one answers my CQ (as usual) I still like to look at SDR sites online to see how far I am making it out. When travelling through Montana and down into Utah my best with the 1/2 wave was 160.3 km for an APRS beacon, and yes this is in flat terrain and ideal conditions. On a practical side however, I guess I figure if I can hit the curvature limit, then when I am closer and in real-use situations (taking to other vehicles), my gear will be optimized. Not sure if that makes sense, but it is part of my rationale.

Around the foothills here I usually get 120 – 140 km for APRS. I probably couldn’t talk to someone at that distance but for APRS it works. That’s also really useful for me, I often go out alone on weekends, and although the goal is to always have your rig prepared for breakdowns you can get stuck. No cell phone signal obviously, and a satellite unit is a bit dramatic for just a local trip. So having an easy way for the family and driving friends to know where I am in case I don’t come back is handy. Being able to email and SMS through APRS is awesome, so if I have to change a tire and will be late I can easily let my wife know.

Btw this is an example of the kind of weekend driving we do up here, with some brief non-APRS VHF use to try and make it slightly relevant. Range required - 200 feet!

 

[prcguy]

I'm not sure why nobody has said it (or if they did and I missed it) but 160km (100 miles, for us yanks) on a 2m mobile setup is not going to happen reliably regardless of which antenna you choose or how much power you use. Just not going to happen.

Some high level repeaters are going to give you that kind of range, and every now and then you might be able to do it on completely flat terrain with no obstructions, but to attempt to get that kind of range with any sort of reliability, especially in mountainous terrain, or simplex, is an absolutely futile effort.

It all depends. I routinely talk 100-125mi 2m simplex from my house on the coast to other base stations in the CA desert with an 8,000ft mountain range between us. Or from my house into Mexico simplex which is even further. This is an everyday occurrence.

 

[alcahuete]

It all depends. I routinely talk 100-125mi 2m simplex from my house on the coast to other base stations in the CA desert with an 8,000ft mountain range between us. Or from my house into Mexico simplex which is even further. This is an everyday occurrence.

I do too....using a base station and base station antenna. Again, this is not going to happen using mobile antennas, 1/4 wave, 1/2 wave, or 5/8 wave. Not reliably. Not simplex.