How high vertically will your signal go with 1/4wave ground plane anntenna

halfwave · Post by **halfwave** » Thu Nov 17, 2016 8:21 am

Hi,

Just wondering if anyone knows if there is a way to work out
how high a signal will go vertically from a 1/4 wave ground plane antenna,
relative to TX Wattage.

I can work out roughly how far it will go horizontally from this
site: http://lrcov.crc.ca/main/index.php.

But I want to know how high vertically it will go, as I have quite a few airplanes
flying over head and want to make sure my signal is not going anywhere near them,
just to be on the safe side. I'm only running low power @ 7 watts.

Any pointers would be appreciated.

teckniqs · Post by **teckniqs** » Thu Nov 17, 2016 10:24 am

The best way to make sure is to use safe equipment.

Analyser · Post by **Analyser** » Thu Nov 17, 2016 11:15 am

teckniqs wrote:The best way to make sure is to use safe equipment.

'Nuff said.

Analyser · Post by **Analyser** » Thu Nov 17, 2016 11:24 am

To answer the question, seeing as it's clear air between you and the planes it will get out a lot further than on the ground.

This is a calculator which will help with the theoretical distance covered but does not take in to account transmitter power or receiver sensitivity. It's just to prove a point.

http://www.vwlowen.co.uk/java/horizon.htm

3M amateur · Post by **3M amateur** » Thu Nov 17, 2016 5:58 pm

halfwave wrote:Hi,

Just wondering if anyone knows if there is a way to work out
how high a signal will go vertically from a 1/4 wave ground plane antenna,
relative to TX Wattage.

I can work out roughly how far it will go horizontally from this
site: http://lrcov.crc.ca/main/index.php.

But I want to know how high vertically it will go, as I have quite a few airplanes
flying over head and want to make sure my signal is not going anywhere near them,
just to be on the safe side. I'm only running low power @ 7 watts.

Any pointers would be appreciated.

Hi,

I am using the same calculation and tested it in practice and it corresponds quite well. A GP antenna signal spreading obliquely upward and is used when you want to send the signal up. So I would in your case using a dipole antenna instead. It sends the signal straight.

Albert H · Post by **Albert H** » Fri Nov 18, 2016 3:54 am

VHF radio signals go where they can see - they always go to the horizon, and a good proportion of your signal will go straight upwards - there's really no way to prevent that.

Remember - VHF always goes to the horizon. A 1 Watt transmitter has exactly the same "range" as a 1000 Watt transmitter - the only difference is how far above the noise floor the signal will be.

radionortheast · Post by **radionortheast** » Fri Nov 18, 2016 9:36 am

hmm sure you mean going up, guess the closer they come the more likely they’ll beable to hear something afew 100 meters over the house!, if you search gp antenna you’ll problaly get to the angle of radiation on ham radio sites, it will be a squashed donaunt

think what your feeling just follows a natural pattern first of all you worry about interference to other fm stations then tv, then aeroplanes

if you had a scanner knew the frequencies used you could just walk afew meters away see if theres anything on the frequencies, but scanners overload easy so you can’t trust them next to a transmitter, think distress frequency is around 120mhz

theres also tetra around 390 mhz,apparently this could fall on the 4th harmonic, dab around 215, tv wich you problaly know, you’ll problaly have a dab radio and tv to check, its easy enough to find the frequencies other wise

halfwave · Post by **halfwave** » Wed Dec 07, 2016 1:49 pm

Hi,

Thanks for all your replies, been busy and only just had chance to check back.

I think getting a scanner and moving away from the site and seeing if the station
is causing interference on the specific aircraft frequencies is the way to go.

Does anyone know of a good cheap (max ~£60) scanner that covers a large frequency range?

NOYB · Post by **NOYB** » Mon Dec 12, 2016 8:20 pm

As an indication, at that power Ofcom expect at least 14dB attenuation at angles above 85 degrees. (I think that's right, my memory might be wrong!). A vertically polarised dipole delivers that, but as you angle it sideways (eg 45 degrees to get mixed pol) then it breaks that limit.

Albert H · Post by **Albert H** » Tue Dec 13, 2016 9:44 am

I was never convinced by "mixed" polarisation. I experimented with every kind of polarisation and found that vertical wins out in most situations. It is worth shaping your coverage - it's better to broadcast into town rather than waste half the expensive RF energy transmitting to farmland!

It's sometimes worth experimenting with different beam angles. I used to use the "MSJ" - modified slim Jim - which is great for very high sites (like up in the Pennines) since it has a lowered radiation angle and covers nearby valleys well. It's often worth experimenting with various types of aerial, and a good aerial is much cheaper than a bigger power amplifier!

A useful tool is a field strength meter - it might not give you actual uV/m, but it can give you indication of relative signal strength. I have a homemade one that gives an audible output - the higher the pitch, the stronger the signal. This plugs into a very low power FM rig so that I can "hear" the field strength as I adjust.

NOYB · Post by **NOYB** » Tue Dec 13, 2016 5:32 pm

I agree Albert - Vertical is best. But if you want to comply with regulations, going mixed allows you to transmit additional power on the horizontal plane.

Albert H · Post by **Albert H** » Wed Dec 14, 2016 2:17 pm

Absolutely - OFCOM have made the mistake with the current round of "low power" FM licences to allow 400W per plane. With a little creative engineering, it should be possible to get a huge field density, receivable with aerials in any direction! I have a funny feeling that they're going to realise exactly what the implications of their specifications are, and limit the ERP and the planar components that are permitted! It's often funny to read their specifications and work out just what you could possibly get away with - their paperwork passes through too many hands and is subject to the whims of their legalistic Civil Servants, who haven't got the slightest clue about engineering.

nrgkits.nz · Post by **nrgkits.nz** » Thu Dec 15, 2016 8:33 am

The sky tower here in Auckland is using circular, looking up at the tower there appears to be 4x 8 stacks of circular panels, each stack placed at 90 degrees around the tower. Then below this there is a 4 stack of dipoles facing the north west for stations using non standard channel spacing. I can often hear slight multi-path distortion when driving around listing to a station on the 8 stack.

Maximus · Post by **Maximus** » Thu Dec 15, 2016 10:47 am

^This reminds me of the Grantham BBC & Lincs fm repeaters (which are needed as the town is down in a bowl type typography). I'd never experienced multipath till walked around the town with a Walkman. The signal was massive but it kept bouncing off the buildings and created some really strange noises. It almost sounded like an analogue tv transmitter breaking through, with fuzzy buzzing and crackling noises. The signal was fine when you headed out of town centre away from the buildings.
The funny things is they're only running 50w.

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