Radial length for scanner antenna?

[brianc33]

I'm going to mount a single band 480MHz scanner antenna on the roof of my house with this Tram ground plane kit

Tram Base Ground Plane Kit | Scanner Master

Tram Base Ground Plane Kit

www.scannermaster.com

Is it useful to trim the radials to my band's quarter wave, or does that not matter for receive-only?

Thanks.

 

[iMONITOR]

I don't know if it makes a big difference as it's a ground plane not a discone. However if you need a ground plane kit for lower frequencies Larsen makes one:

144~500MHz NMO Ground Plane Kit

Larsen Base Conversion Kit

 

[Ubbe]

As they are only thin rods acting as ground plane they will work best at their 1/4 wave tuned frequency. If you have a solid metal sheet as ground plane it should be at least 1/4 wave in radius but are not tuned so can be larger in size and will improve the ground plane.

What's the exact antenna you are going to use? Ground plane radials should be tilted down to match the impedance and pull down the directional loob lower to the horizon.

/Ubbe

 

[brianc33]

Thanks for the advice. This will be used with a simple straight vertical thin rod cut to 1/4 wave of 480. So it sounds like I should cut the radials to the same length as well as angle them down to get the impedance right. I don't have an antenna analyzer, is there another way to determine that the right impedance has been achieved?

 

[Ubbe]

The impedance change with frequency and at the exact frequency a 1/4 whip are tuned to will theoreticly be something like 35 ohm and when folding down the ground radials it will increase and fully verticly down will be something like 70 ohm.

When you have one element straight up and one straight down vertical it will be a dipole and the direction loob points more or less to the horizon. When having ground plane rods angled horizontally or a metal surface like a car roof it will also tilt the direction more up in the sky and you loose range at the horizon where the transmitters are positioned.

The impedance to frequency diagram looks something like this: (It's a 155MHz antenna and not 145Mhz)

/Ubbe

 

[jonwienke]

"Lobe".
Not "loob".

 

[prcguy]

If there is a chance you might use the ground plane on VHF you can cut the radials to 3/4 wavelength on UHF and it will work fine on UHF and they will be a 1/4 wavelength on VHF. You don't really need to bend the radials down, the mobile antenna you are putting on it was made to go on a flat sheet metal car body part, so it will tune up just fine stock and 90deg to the vertical whip.

 

[brianc33]

This is great information, thanks all.

 

[KO4IPV]

I have the Larson ground plane kit , I installed on my roof with radials bent 70 degrees it works fine , cutting the radials was not done and it worked out perfect , reception was great , since have replaced with the Omni-X much higher off ground . and no sheet metal was needed with the Larson kit.

 

[Ubbe]

"Lobe".Not "loob".

Thanks. We have a hockey player named Loob and I have probably seen that too much in print that it got stuck and it is also pronounced in the exact same way.

/Ubbe

 

[jonwienke]

Thanks. We have a hockey player named Loob and I have probably seen that too much in print that it got stuck and it is also pronounced in the exact same way.

/Ubbe

LOL. It can get stuck in your autocorrect, too.

 

[Ubbe]

The spell check in Firefox that I use only do swedish and I would have to switch to Chrome or something to get a working dual language autocorrect. Some web forums have an inbuilt spell check but not RR, or I haven't found where to enable it.

/Ubbe