Should i cut down the ground plane radials?

[sking128]

I have a Tram Model 1465 Base Ground Plane Kit that I am going to use with an NMO mount antenna for a DMR repeater at work. Repeater TX is 469.2875 an the Reaper RX is 464.2875. The kit comes with 20.5" radials. do I need to cut them down, like the Antenna, or should I leave them alone. If I need to cut them, then what formula should I use to determine length? And advice would be helpful.

Thanks,
Steve

 

[prcguy]

The kit comes with the longest elements you would use around 136MHz and you would cut them shorter for higher freqs. I would expect a cutting chart to come with the GP kit and the radials should end up in the 6" range.

I have a Tram Model 1465 Base Ground Plane Kit that I am going to use with an NMO mount antenna for a DMR repeater at work. Repeater TX is 469.2875 an the Reaper RX is 464.2875. The kit comes with 20.5" radials. do I need to cut them down, like the Antenna, or should I leave them alone. If I need to cut them, then what formula should I use to determine length? And advice would be helpful.

Thanks,
Steve

 

[sking128]

The kit comes with the longest elements you would use around 136MHz and you would cut them shorter for higher freqs. I would expect a cutting chart to come with the GP kit and the radials should end up in the 6" range.

Yeah, I was expecting a chart too, but alas, there was none.

 

[Ubbe]

The antenna are designed to be used on car roof, a full metal sheet. To make best use of the ground plane kit you would want the elements to be tuned to a 1/4 wave of the frequency, which is that 6" length. If you can mount 12" sheet metal between the NMO mount and the mast mount it would be even better.

/Ubbe

 

[W5lz]

A couple of 'general' things about radials (meaning they're wrong half the time).
Radials too long seldom make a really significant difference. Mainly in the input impedance and just how 'resonant' the antenna is (broad or narrow). The number/size of radials typically furnished with any antenna is the -minimum- required to make it 'work'. Also be aware that changing those lengths can mean re-tuning the thing. If the manufacturer of the thing used any thought in designing it, they have already done the experimenting with number and lengths for a 'broad' range of frequencies. They usually have better knowledge in that than I do...

 

[merlin]

For receiving applications, it makes no difference.
Transmitting, you are best off cutting the antenna for the best TX SWR.
A bad SWR will cause bad problems with duplexers.
There are lots of online calculators for this, consider ground radials usually cut 20% longer.