Antenna length/cutting question

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JoshuaHufford

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I just bought this mount,

https://www.theantennafarm.com/catalog/diamond-k515snmo-4992

And I picked up this antenna for it used at very good price,

https://www.theantennafarm.com/catalog/pctel-maxrad-mhb5802-755

This is going on a 2014 Subaru Outback. I do not want to do a permanent NMO mount on this vehicle, and I don't really want to use a Mag Mount frequently either so this is what I'm going with. I know this is not ideal for the best reception but for this vehicle it is what I want, and this will give me the ability to roll down the window flip the antenna down and pull in the garage, I can't do that with any other antenna, my garage is pretty low and the car is pretty tall.

My question is on cutting the whip, I measured it right at 37", and if I'm using the calculator correctly would put this at 151.8MHz, my primary interest is the Railband at 160-162MHz.

So should I cut it down? I want to give it every advantage that I can.

Is 35" the correct length for centered at 161MHz?

Do I measure the length of the whip after it is installed in the coil or the length of the whip when it is out of the coil?

And it seems pretty straight forward to cut it but I've never done this before. Best too to use?

As always thanks for the good info and help.
 

prcguy

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Antenna resonance can be influenced by where its mounted and its surroundings. I would get/borrow an antenna analyzer and tune it specifically to your desired frequency instead of relying on a cutting chart.
 

KE5MC

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Antenna is already in your area of interest on receive. Trimming antenna impacts transmit more than receive. Once you get the analyzer as prcguy suggests, I would temporally replace the antenna wire with a length of straightened coat hanger. Your antenna has the "advantage" of not having a curly cue inductor fabricated in the length. I expect that the coat hanger wire is smaller diameter allowing as an easy substitute. Establish baseline measurement with current antenna. Replace with same length coat hanger wire. I'm not expecting you will see a change. Cut and measure until you get results you are looking for. If cut to short replace coat hanger wire with last known good length, reduce cut length increment. Transfer length to original antenna and cut. Measure and compare to last good coat hanger length measurement.

Report back on how it turns out.
 

cmdrwill

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I would hope that someone who has an antenna analyzer and is close by would help the OP 'tune' his new antenna. And YES 'tuning' the antenna does make a difference.

I do that for our local friends. Yesterday I found that the antenna was the wrong one, not what he ordered. So of course it would not tune up even tho it looked just like the one in the ad. But it still 'received' 'OK".....
 
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Ubbe

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Use a crocodile clip or two on the antenna to lenghten it to check if it needs to be longer. Carefully listen to a weak transmission, I beleive railroad are all analog, and add a crocodile clip and compare the signal strenght. Then remove clip and listen and then add two crocodile clips in series and compare the signal strenght. If the signal goes worse with one clip and even more with two you can cut the element shorter by one clip lenght and try the test again.

alligator-clips-for-earthing-leads.jpg



/Ubbe
 

jwt873

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As KE5MC and radioman2001 point out in the posts above.. If it's just for receiving, SWR isn't a critical thing.

Larsen even supplies cutting charts with their antennas for people who don't want to bother measuring SWR. The chart in this link is for an antenna similar to the Maxrad antenna you have. https://www.pulselarsenantennas.com/wp-content/uploads/2016/09/NMO-VHF-Series-08-12.pdf - Note that the antenna has a bandwidth of 6 Mhz at 1.5:1 and 16 Mhz at 2.0:1

Of course there's nothing wrong with using an analyzer or an SWR bridge.. It's a learning experience.
 
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