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Old 12-08-2018, 02:37 PM
greasemonkey331 greasemonkey331 is offline
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Join Date: Dec 2018
Location: Newport N.C.
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Default Vhf antenna help

I have no experience with antenna building, and know very little about them.

Can anyone tell me if a scanner antenna, will transmit for vhf??

I made this and it receives great,
https://images.google.com/imgres?img..._M&w=506&h=754

I want to make a vhf antenna that will reach approx 30 miles on land, for use in hunting club.

Thanks in advance for your help.

Last edited by greasemonkey331; 12-08-2018 at 02:41 PM.
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Old 12-09-2018, 07:54 AM
paulears paulears is offline
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Default Re: Vhf antenna help

The real issue with the difference between scanner antennas and transmit antennas is their impedance. For receive, it doesn't matter - so having an antenna design with multiple different length elements produces results that are a combination of lots of useful efficient 'collectors' of RF energy. Transmit is more of an issue because the transmitter needs to see an antenna that has as close to an impedance of 50Ohms. If the antenna is 200Ohms impedance at maybe marine band, 50 Ohms at the ham band and 150 Ohms in the aircraft band - it will transmit happily on the ham band, but even if the radio can transmit elsewhere, the wrong impedance results in low power and even damage in some cases, as what the radio sticks out, comes right back top the cable!

Antennas have properties too - they can have a nice 50Ohm impedance and be totally rubbish at getting RF out and collecting it back. We determine how good they work with a gain figure. If you end a bit of coax cable, and take the inner conductor up with some stiff wire around 19-20" long, and take an identical length and go down from the coax braid, you have created the 'standard' ? wave antenna design. 50Ohm, and having a gain figure of 0dB - compared to a ? wave antenna (that it is!) It will function on the 2m ham band fine. Trim the ends to make it shorter and it works on marine band. Make it longer and it's good for aircraft band.

that magic 0dB figure drops when the antenna works away from it's design frequency - so the 2m cut antenna is less good at marine band and aircraft band. The little stubby bits on the scanner antenna trick the design into covering more than one band as it is lots of different lengths - each adding and subtracting in complex ways. No good for transmit though.

You mention 30 miles. This depends not on the antenna, but geography - your antenna na on the roof of your house might go 40 miles if you live on the top of a hill, and might struggle to do 4 miles if you live in the valley between two hills. Topography is everything. You can get antennas (although not generally ones with wide band performance) that have better performance, but in general. Height is the key to distance.

You did realise you posted in the VHF marine section? Not really the place for your planned activity.

Last edited by paulears; 12-09-2018 at 07:57 AM.
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Old 12-11-2018, 06:15 AM
greasemonkey331 greasemonkey331 is offline
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Location: Newport N.C.
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Default Re: Vhf antenna help

I apologise if this is in the incorrect place, I have never posted anything like this before.

we use vhf radios for our hunting club and I am trying to find a buildable antenna for decent output and distance, it is flat land 5k acres but wooded, not sure if this affects signal or not. Thanks for the explanation and any help. Greg.
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Old 12-11-2018, 10:34 AM
paulears paulears is offline
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Default Re: Vhf antenna help

As I said Greg, it depends on the topography - but also the system. If you want direct radio to radio comms, then 5K distance is going to be difficult - sometimes even a mile will not work. Signal gets attenuated as it passes through anything other than free space. Hence why 1W will get you too the European Spec station from a handheld - but that handheld can't manage a mile through woodland. Anything in-between is bad. Steel, concrete, rebar in the concrete - sap in the trees, water in people's bodies - all get in the way. You can use a repeater, licence permitting, and this will increase the range - and is where a high performance antenna will help. As to you building one? depends on your skills and test equipment. Gain=more complexity, greater understanding of the physics and the ability to test accurately.

If the idea is people with a radio attached to webbing, running around the woodland of 5000 acre size, with no 100ft tower and a repeater on a high point somewhere central, then forget it - it's simply impractical to bend physics in this way.
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