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

[jeepsandradios]

Not sure if anyone else shared this but your tests are not accurate with APRS as it will beacon at different rates and if there is a stronger signal hitting a digi you will miss a TX. While a good test it can be misleading. I have a 14 miles section of road from my house to town. Straight road up and down hills and I can lay day after day over a map and every beacon and position is different.

 

[AK9R]

While a good test it can be misleading.

Let's say that running this sort of test using APRS is not conclusive. You kinda have to run multiple tests and average the results. I sometimes think that folks believe APRS to be some sort of deterministic communication medium. It's not. In busy APRS area, there will be lots of collisions between APRS packets and some packets will just not be heard.

 

[nd5y]

Let's say that running this sort of test using APRS is not conclusive. You kinda have to run multiple tests and average the results. I sometimes think that folks believe APRS to be some sort of deterministic communication medium. It's not. In busy APRS area, there will be lots of collisions between APRS packets and some packets will just not be heard.

That and it uses AFSK tones on FM. That doesn't work well with weak noisy moving signals where FM voice would be perfectly readable.

 

[Zakowsky2]

I just wanted to do a long term follow up on my antenna selection, mobile, primarily for VHF. I finally got my Landcruiser back on the road (back off-road to be accurate) and just bought a Comet CA-2X4SR NMO. I repaired my K515SNMO Diamond Roof Rack mount (which had the cable and the actual NMO mount part ripped out) using a Wilson NMO mount for 3/8” holes – Diamond sells a replacement cable and mount for it but the cable is a bit short (10 feet and I need 13 feet), and the Wilson was 1/3 the price. I actually like the Wilson, cable seems better and it is sturdy once mounted. Took a bit of grinding to make it fit but not too bad.

Anyway the reason I went for the CA-2X4SRNMO was even though it is advertised as needing a ground plan, if you look at the actual specs it is 2 x 1/2 wave on 2 meters and 3 x 5/8 wave on 70 cm. Since I usually only use VHF (but want the option of other bands) I thought maybe it would work well for 2 m without a good ground plane and only suffer on UHF. Nobody at the stores really knew so I gave it a try.

First impressions are it seems pretty good. In my albeit flawed APRS test this was the result:

So technically the CA-2X4SR could be the best yet. It works on VHF very well under most condition, better I would say then my SBB5 style 1/2 wave I was using before. So maybe for VHF it doesn't need a great ground plane. But I was a bit disappointed in its performance in deep canyon-like terrain – the picture doesn’t really show it but on other parts of the drive it failed to hit repeaters that the 1/4 wave and maybe even the SBB5 single element 1/2 wave could.

Given how I have to mount it, I think I will stick with the CA-2X4SR for a while, and carry my 1/4 wave mag mount just in case.

 

[KC3ECJ]

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 can happen if its SSB.

 

[alcahuete]

It can happen if its SSB.

1. Most people don't run VHF SSB mobile.
2. Most SSB is horizontally polarized. Mobile is generally not.

 

[ki6zup]

For all of you keyboard warriors.........

RF bonding is the key to a good mobile installation.
A simple K400 lip mount and some well placed 1" braid will go a lot farther than a 1000.00 antenna taped to an insulated roof rack.

visit this site, read and learn.......

KØBG.COM

A Web Site For Mobile Amateur Radio Operators

www.k0bg.com

its a hobby....its supposed to be fun and educational
mobile 2m SSB specialist ..just waiting for the next tropo event

 

[kayn1n32008]

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

0dBd with out a groundplane, 2.5dBd with a proper groundplane.

 

[kayn1n32008]

On the left is my APRS track, 30 second intervals, 80 watts, with the antenna in the center of the roof.

Pretty much wasting electrons at 80w. Your performance is going to be the same @50w, and the radio will run cooler.

In BC, Yukon and Alaska, I would run a 1/4 or a 1/2 wave. Yukon has a decent repeater network, and the Prince George group has a decent network in northern BC.

Having grown up in BC, and traveled extensively in BC, I usually just stick to a 1/4 wave and a 25w or 50w mobile.

 

[kayn1n32008]

I just wanted to do a long term follow up on my antenna selection, mobile, primarily for VHF. I finally got my Landcruiser back on the road (back off-road to be accurate) and just bought a Comet CA-2X4SR NMO. I repaired my K515SNMO Diamond Roof Rack mount (which had the cable and the actual NMO mount part ripped out) using a Wilson NMO mount for 3/8” holes – Diamond sells a replacement cable and mount for it but the cable is a bit short (10 feet and I need 13 feet), and the Wilson was 1/3 the price. I actually like the Wilson, cable seems better and it is sturdy once mounted. Took a bit of grinding to make it fit but not too bad.

Anyway the reason I went for the CA-2X4SRNMO was even though it is advertised as needing a ground plan, if you look at the actual specs it is 2 x 1/2 wave on 2 meters and 3 x 5/8 wave on 70 cm. Since I usually only use VHF (but want the option of other bands) I thought maybe it would work well for 2 m without a good ground plane and only suffer on UHF. Nobody at the stores really knew so I gave it a try.

First impressions are it seems pretty good. In my albeit flawed APRS test this was the result:
View attachment 162746
So technically the CA-2X4SR could be the best yet. It works on VHF very well under most condition, better I would say then my SBB5 style 1/2 wave I was using before. So maybe for VHF it doesn't need a great ground plane. But I was a bit disappointed in its performance in deep canyon-like terrain – the picture doesn’t really show it but on other parts of the drive it failed to hit repeaters that the 1/4 wave and maybe even the SBB5 single element 1/2 wave could.

Given how I have to mount it, I think I will stick with the CA-2X4SR for a while, and carry my 1/4 wave mag mount just in case.

I'd stay far away from Comet antennas. For best results stick with mono band antennas.

 

[Zakowsky2]

So I was not going to post this update at first figuring there wouldn't be much interest, but as mentioned

its a hobby....its supposed to be fun and educational

That is a great site you linked btw, it is great to read such in-the-field style knowledge. And yeah, this is just supposed to be a fun hobby. Between cell phones and satellite messaging, it’s really a small population that use this tech anymore. Personally I just enjoy taking my APRS route and seeing which repeaters I can hit with different antennas mounted differently on the truck. I know the spots where it is hard to get signal out of, and if I can set things up and make it out, as a hobby that is more the goal than Search and Rescue functionality if that makes sense. And I do like the APRS as a metric – if I can hit the repeater the packet almost always gets through – traffic is not an issue here, because there are only about 10 people in the entire city that still use APRS, and usually only one at a time! So while I do use VHF for real when Overlanding, the “fun” in it from a geek stand point maybe is enjoying optimizing equipment.

Pretty much wasting electrons at 80w. Your performance is going to be the same @50w, and the radio will run cooler.

In BC, Yukon and Alaska, I would run a 1/4 or a 1/2 wave. Yukon has a decent repeater network, and the Prince George group has a decent network in northern BC.

Having grown up in BC, and traveled extensively in BC, I usually just stick to a 1/4 wave and a 25w or 50w mobile.

I am looking forward to heading up there and seeing how it does. For the 80W, I actually prefer to use my FT5DR in the truck; its small, easy to mount and use, and the screen is not that much smaller than the mobile units. Plus I carry it while hiking with a 1/2 wave Long Ranger antenna so it works well for me. In the Landcruiser I have a coax that runs back to a RM Italy power amp, so no stress on the radio. Regarding the Comet, although I had a mono band VHF 1/2 wave before, it does seem that the 2 x 1/2 elements in the Comet might work better. At least not worse.
So on to my update. Watching my APRS track as I drive I saw that as I changed direction there was a big change in successful contacts. As many have mentioned mounting the antenna in the middle of the roof rather than the side is important, but how important I wanted to test. So I shifted the mount from the side to a cross bar on the rack and wow, a huge improvement! On stretches that I would get like 3 hits, it went up to 15 hits! The route I posted above now has 58 total contacts. So directionality, with a 2 x 1/2 wave antenna on a roof rack on a Landcruiser is a major issue. I have to figure out some way to mount this in the center with my gear loaded. And then work on grounding straps, or go right into the roof if I can modify things to permit that.

 

[kayn1n32008]

So I was not going to post this update at first figuring there wouldn't be much interest, but as mentioned

That is a great site you linked btw, it is great to read such in-the-field style knowledge. And yeah, this is just supposed to be a fun hobby. Between cell phones and satellite messaging, it’s really a small population that use this tech anymore. Personally I just enjoy taking my APRS route and seeing which repeaters I can hit with different antennas mounted differently on the truck. I know the spots where it is hard to get signal out of, and if I can set things up and make it out, as a hobby that is more the goal than Search and Rescue functionality if that makes sense. And I do like the APRS as a metric – if I can hit the repeater the packet almost always gets through – traffic is not an issue here, because there are only about 10 people in the entire city that still use APRS, and usually only one at a time! So while I do use VHF for real when Overlanding, the “fun” in it from a geek stand point maybe is enjoying optimizing equipment.


I am looking forward to heading up there and seeing how it does. For the 80W, I actually prefer to use my FT5DR in the truck; its small, easy to mount and use, and the screen is not that much smaller than the mobile units. Plus I carry it while hiking with a 1/2 wave Long Ranger antenna so it works well for me. In the Landcruiser I have a coax that runs back to a RM Italy power amp, so no stress on the radio. Regarding the Comet, although I had a mono band VHF 1/2 wave before, it does seem that the 2 x 1/2 elements in the Comet might work better. At least not worse.
So on to my update. Watching my APRS track as I drive I saw that as I changed direction there was a big change in successful contacts. As many have mentioned mounting the antenna in the middle of the roof rather than the side is important, but how important I wanted to test. So I shifted the mount from the side to a cross bar on the rack and wow, a huge improvement! On stretches that I would get like 3 hits, it went up to 15 hits! The route I posted above now has 58 total contacts. So directionality, with a 2 x 1/2 wave antenna on a roof rack on a Landcruiser is a major issue. I have to figure out some way to mount this in the center with my gear loaded. And then work on grounding straps, or go right into the roof if I can modify things to permit that.

I wouldn't worry about ground straps on VHF, it's not going to gain you anything. Ground plane and DC ground are different, and not really related.

 

[JustinWHT]

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.

You might be able to key up the repeater with 80 Watts, but you may not hear it talk back to you.

We had 100 Watt UHF Motorola Micros and a few 20 Watt Mocoms that got close to same repeater coverage due to the terrain.

Remember... For every 6 dB (or four times) increase you double your range assuming flat terrain.

 

[JustinWHT]

Some high level repeaters are going to give you that kind of range,

There's a 2 M repeater on the [current name IDK] on Dallas's tallest 72 story (900 ft) building that I can't hit from 40 miles out.

 

[prcguy]

You might be able to key up the repeater with 80 Watts, but you may not hear it talk back to you.

We had 100 Watt UHF Motorola Micros and a few 20 Watt Mocoms that got close to same repeater coverage due to the terrain.

Remember... For every 6 dB (or four times) increase you double your range assuming flat terrain.

That would be using the square law backwards and it doesn’t work that way. The law which includes photons and RF states every time you double the distance the level will drop by four times or 6dB, assuming line of sight and no multipath or reflections that can skew the results anywhere from adding 6dB to attenuating up to 20dB due to in or out of phase reflections. For earth to space where there are no reflections, the square law works perfect.

Trying to use it backwards like increasing your power by four times or 6dB will provide the same receive level at twice the distance in space with no reflections but on the ground you will have reflections and multipath that will skew the results and it will usually not double the distance due to terrain, earth curvature, etc.

If you use online free space antenna/power/distance calculators, they do not take multipath into account and will never give an accurate result except in space or earth to space. That’s why they are called “free space” calculators.

 

[JustinWHT]

Remember... For every 6 dB (or four times) increase you double your range assuming flat terrain.

Let me parse that for correctness and clarity. I actually dozed off and dropped my phone.

 

[JustinWHT]

Let me parse that for correctness and clarity. I actually dozed off and dropped my phone.

Basic inverse square law - every time you double or half your distance, the signal level decreases 4x or increases 4x. It follows if you increase signal by 6 dB or 4x you double your distance. All that is assuming flat terrain with negligible multipath effects using the Longly-Rise propagating model.

Now to throw a wrench into conventional thinking.
RF engineers kneel at the alter of the Friss Equation and assume the higher the frequency (or shorter the wavelength), the free space attenuation increase. No, it does not - free space attenuation is not frequency/wavelength dependant. I should start a new thread for that.

To throw in another wrench for good measure. RF engineers also put too much faith in the Fresnel zone predictions for reflections off the ground. There has been only once in my life I found it successfully explained one propagation problem. Likewise I should start a new thread.

 

[JustinWHT]

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?

Back in the (ahem) "old days" antenna gain was referenced to a simple dipole. A Decibel Antenna DB 224, a four dipole VHF antenna, was advertised as 6 dB gain. The dipole reference worked for decades until....

The FCC introduced the isotropic antenna standard. Manufacturers jumped on this because it allowed them to advertise a zero or unity gain antenna as 2.15 dB gain antenna.

Thus an 18" VHF mobile whip of zero or unity gain was advertised as a 2.15 dB gain antenna.

It's standard practice, or should be, to differentiate the comparison by using dBd (over ground plane or dipole) or dBi (over the mythical isotropic) gain. But people are numerically challenged and will purchase an advertised 2.15 gain antenna over a 0 dB gain identical antenna.

Short form: 2.15 dBi = 0 dBd

Therefore to answer your question...a magnet mounted 1/4 wave antenna will.have zero or unity gain, not 2 dB gain.

 

[prcguy]

Basic inverse square law - every time you double or half your distance, the signal level decreases 4x or increases 4x. It follows if you increase signal by 6 dB or 4x you double your distance. All that is assuming flat terrain with negligible multipath effects using the Longly-Rise propagating model.

Now to throw a wrench into conventional thinking.
RF engineers kneel at the alter of the Friss Equation and assume the higher the frequency (or shorter the wavelength), the free space attenuation increase. No, it does not - free space attenuation is not frequency/wavelength dependant. I should start a new thread for that.

To throw in another wrench for good measure. RF engineers also put too much faith in the Fresnel zone predictions for reflections off the ground. There has been only once in my life I found it successfully explained one propagation problem. Likewise I should start a new thread.

Oh really? Looking at purely wavelengths then no, frequency does not enter the free space calculation. If you are 1,000 wavelengths away at 100MHz you will have the same loss as if you are 1,000 wavelengths away at 400MHz and so on. Problem with your statement is given the same distance in feet, there will be 4X more wavelengths at 400MHz as there will be at 100MHz therefore more loss at 400MHz at the same distance in feet.

Over the years my employment had me taking many field strength measurements and one was for the largest satellite company in the US which I worked for. It seems a rival company got FCC permission to transmit terrestrially within a satellite downlink band, wreaking our customers reception within their terrestrial towers. I was dispatched on a mission to measure our customers signal degradation and measure precise filed strength of the enemy’s field strength at a bunch of locations.

Being in Ku band and transmitted from towers there was a big problem with ground reflections and multi path and my solution was to take a dozen or more measurements vertically over about 100 wavelengths to capture various points of multi path and resulting gain or loss. BTW 100 wavelengths is about 7.8ft at this frequency and I had about 12ft of telescoping mast for the measurements. The resulting measurements were all over the place (as I expected) ranging from up to 6dB higher to 10-15dB lower than calculated for the distance to tower measured by laser rangefinder. I’ve had similar results at lower frequencies during other field tests. Botton line is multipath reflections do add or attenuate depending on if the r

 

[JustinWHT]

Problem with your statement is given the same distance in feet, there will be 4X more wavelengths at 400MHz as there will be at 100MHz therefore more loss at 400MHz at the same distance in feet.

While I don't know why the number of wavelengths enters the picture, there will not be any more (free space loss I'm assuming your referring to) comparing 100 and 400 MHz. What equation did you use?

Though is more loss at 400 MH.when comparable gain antennas are used , it has nothing to do with free space loss and in practical applications its easily compensated such that the total PtP (antenna to antenna) signal levels are greater as you increase the frequency.