Don’t have to trim Laird BB1443S for RX only ?

[mbnv992]

Figure I’d run this past the experts here. Friend of mine has a Laird 5/8 wave VHF 144-174 MHZ BB1443S on the way. It’s going to a Spectra that’s programmed anywhere from 150 all the way up to the WX bands. All RX only. He wanted to have this antenna cut and tuned for the 155ish MHZ since that’s where most of the frequency’s are in ( and also to trim it down so it’s not the full 49” in length ).

My local radio / Motorola authorized dealer said that with this particular antenna it doesn’t need to be cut or tuned at all since it’s rated for the 144-174MHz range. However - the specs on this antenna clearly say that it only has about 6mhz of usable bandwidth once it’s cut and tuned.

So I guess my questions are

1. - Does this antenna have to be tuned and cut just if it’s used as RX only ?

And 2 - if I do cut it to the cut chart ( which would put it at about 43 inches ) would it effect the performance for the higher WX frequencies ? Everything I read said when you cut the antenna it’s just basically dials in the TX performance and has nothing to do with RX only frequencies. So if it’s cut would it effect the other frequencies that are outside of that 6mhz ?

Thanks !

 

[GTR8000]

While antenna trimming is generally done to achieve a better SWR match for transmitting, there is no harm in trimming it for RX only applications.

If you leave the B1443 uncut, it will be resonant more at the lower end of the coverage, i.e. 144 MHz. Since your friend primarily cares about 150 MHz and higher, shortening the whip will help center that 6 MHz of bandwidth more towards the sweet spot he's listening to. And of course the shorter the whip, the better the resonance on higher frequencies, so cutting it will help with the 162 MHz frequencies, not hurt.

Make sure you follow the correct chart for the model with the spring, since it's a B1443S, otherwise you'll wind up leaving it too long. According to the old Laird chart I have, the B1443S should be cut to 41" for 155 MHz (43" would tune it to around 149 MHz). Double check that against the latest cut chart supplied with the antenna to make sure it's still current. Measure twice, cut once.

 

[mbnv992]

Yep you are correct. 155 would be around 41”. I also read to trim a few inches longer than the cut chart recommends and then to get its tuning Info checked at a radio shop and then they could cut 1/4 inch off at a time to get a proper rating. I don’t necessarily trust my local MSS if they are telling me to keep it un-cut and it doesn’t need anything haha
So - just trim to the chart and it should be fine ?

 

[mass-man]

That cut it long advice is for tuning for transmit...cut it at 41" and that should put you at 155mhz, per the chart. You should be fine for the 162 WX channels.

 

[mmckenna]

I also read to trim a few inches longer than the cut chart recommends and then to get its tuning Info checked at a radio shop and then they could cut 1/4 inch off at a time to get a proper rating.

No, you don't need to take it to a shop. Cut it per the correct cut chart length. That'll be close enough for what you are doing. Do not pay a shop to tune the antenna for you if all you are doing is receiving. That would be a waste of money and have little, if any, impact on performance. The cut charts usually get you close enough.

My local radio / Motorola authorized dealer said that with this particular antenna it doesn’t need to be cut or tuned at all since it’s rated for the 144-174MHz range.
----------------
I don’t necessarily trust my local MSS if they are telling me to keep it un-cut and it doesn’t need anything haha
So - just trim to the chart and it should be fine ?

"Motorola authorized dealer" use to mean something, sadly it doesn't any more. You are wise not to trust this shop. Either you talked to someone who has no idea what they are doing, or they really are lacking in the knowledge department. I'd seriously wonder how many public safety agencies use this shop and are running around with improperly set up equipment.

 

[GTR8000]

Remove the whip from the spring using a hex (Allen) wrench to loosen the set screw. Measure from the top (ball end) down, marking it where it needs to be cut at the end that goes into the spring. Put it in a vice, and cut it at the mark using a fresh bi-metal hacksaw blade. Stick it back in the spring, tighten set screw snug but not over-tight, and done. That simple, don't pay anyone to cut it. It does not need to be exact for RX only, just get it in the neighborhood and it'll be fine.

 

[mbnv992]

Cool thank you ! I don’t have a metal saw but I do have some nice bolt cutters that should cut it just fine I would think.

 

[mmckenna]

Cool thank you ! I don’t have a metal saw but I do have some nice bolt cutters that should cut it just fine I would think.

Those will work.
So will a file if you can file a notch and snap it off with a pair of pliers. (if you have a file, file the burrs off the end after you cut it, too)
A large pair of Vise-Grip pliers that have the wire cutter section in the jaws works pretty well, too.

 

[KF5LJW]

Being RX only, there is no need to prune the antenna. Only becomes important if you use it to TX. If your interest lays in broadband RX only your money is much better spent buying a Dicone antenna. A Discone can easily cover VHF to 2 GHz, and can be used for TX if you are willing to pay up. RX only is cheap.

 

[mbnv992]

Thank you for all the help ! Yep, no TX’ing with this one.

Speaking of, I have a Laird BB4505CS which is a 5/8 over 5/8 colinear UHF 450-470mhz antenna that I switch out sometimes for a 1/4 wave. I was also told with this antenna since it has a usable bandwidth that’s already tuned for 450-470mhz that I do not need to cut or tune this antenna ( this was also verified by calling The Antenna Farm, where I bought this antenna ) and the documents I’ve read on it said it’s designed to have a non-tuned wider bandwidth of the full 450-470. Yet when I got the antenna in, it had a cut chart on it. This one will be used for GMRS and RX only on mainly 460 frequencies. Should I cut this one too ? I don’t have a SWR meter but it performs very well so far on both TX and RX and has slightly better reception than the 1/4 wave I swap it out with from time to time.

 

[GTR8000]

it performs very well so far on both TX and RX and has slightly better reception than the 1/4 wave I swap it out with from time to time.

You answered your own question. If it ain't broke...

 

[mass-man]

Trimming a 5/8wave antenna is tricky. By design they are broad banded!