Conflicting whip lengths everywhere for 155Mhz, HELP!

[2ahuman1791]

I bought a Laird B132S mag base antenna for VHF communications on the 155mhz MURS bluedot and green dot freqs.
My antennas overall height (the antenna to the part that unscrews from the mag base) is 23" inches. My antenna whip measured from the top of the spring to below the ball tip is 18 and 1/4" inches. with the ball tip its 18 and 1/2" inches.
Ive been down a K hole of conflicting info on what length, leaving the ball tip, not leaving the ball tip...but mostly the length to tune it to 155mhz. Based on the PDF tuning chart it says I need to cut it to 16 and 1/8" inches. If you ask the internet youll get 3 to 6 different answers, including calculators. Sources on internet claim wildly different lengths. from 42" all the way down to 17" and in-between.
Heres a photo of my antenna. Ive read you measure from under the ball to exactly where the antenna enters the spring housing with the set screws. Others say measure the ball too. Some say not to. There is no consensus on 155Mhz for metal chrome whips if you search long enough. likewise for where to measure and cut.
Can someone please help me figure out the length on this thing? Its the B132S Laird antenna. it has 2 Allen set screws that hold whip.

 

[2ahuman1791]

heres the PDF for this antenna but mine came 18 1/4 inches. not 21" like online sales say it is. Is 3" inches buried in the base?

 

[kf8yk]

Here's the PDF for this antenna but mine came 18 1/4 inches. not 21" like online sales say it is. Is 3" inches buried in the base?

Remove the whip from the spring. Per the cutting chart, cut it for 16-1/8" as measured from the top of the ball to the end of the whip. Once reinserted in the spring the antenna will be about the 18" length for a resonate 1/4 wave antenna at 155 MHz.

leaving the ball tip, not leaving the ball tip

VHF 1/4 wave antennas are quite broadband, a difference this small in whip length is not critical.

 

[nd5y]

The spring is part of the antenna.
The metal parts inside the antenna base between the spring and the center contact of the antenna mount are part of the antenna. You have to take that into account if you don't have a cutting chart or equipment to actually measure where the antenna is resonant and the bandwidth.

On the antenna in the picture the electrical length of the antenna is from the top of the ball to probably the bottom plastic edge of the base between the logo/lettering and the threaded metal ring.

 

[mmckenna]

I bought a Laird B132S mag base antenna for VHF communications on the 155mhz MURS bluedot and green dot freqs.
…. Sources on internet claim wildly different lengths. from 42" all the way down to 17" and in-between.

The B132S is a 1/4 wave antenna.

"Sources on the internet" is your problem. 42" isn't 1/4 wave at 155MHz. Sounds like the spec for a 1/2 wave antenna. When looking for cut info, always go to the manufacturer and make sure you are looking at the correct antenna model, and always make sure the cut chart is for the spring base or non-spring base depending on what you have. As said above, the spring is part of the radiating element and needs to be included in the overall length. The Laird cut chart will include that in their specs.

Ive read you measure from under the ball to exactly where the antenna enters the spring housing with the set screws. Others say measure the ball too. Some say not to.

The ball on the end is part of the antenna and should be included in the length.
The cut charts will be for the total length of the whip when it's removed from the mount.

I'd remember those webpages that told you not to include the ball, or told you to measure to where the whip enters the base, and avoid them at all costs. There's a lot of bad info out there, especially when it comes to antennas.

As mentioned above, 1/4 wave antennas are very broad banded and will cover just about all the VHF band with low SWR, even if your cutting in not exact. I've swept 1/4 wave antennas with $20K analyzers and they all perform pretty much the same. A basic 1/4 wave like that will have 2:1 or less SWR across the VHF band from 144 up to 174MHz.

If you really want to split hairs and get anal about all this, get one of the NanoVNA's and you can test yourself, but it's not necessary. Get it close and enjoy the antenna.

 

[Ubbe]

Antenna lengths are measured from the ground plane to the tip. The ground plane for a magnet antenna are a car roof, or the metal sheet you use as a ground plane indoors. If you only use the magnet without any metal below it the antenna works much different and the direction pattern points up in the sky as the magnet alone doesn't have enough area to work as the ground plane the antenna and also the cutting diagram where designed for.

That magnet base are 1,5 inch thick and the cutting chart calculate with that base and show the length for just the whip alone and the lengths then match up exactly with a a 1/4 wave whips total length to its ground plane.

/Ubbe

 

[Golay]

Like mmkenna is saying, the difference in length is due to what kind of antenna it is. A simple quarter wave with no base coil is going to be around 18". My 5/8 wave antenna for two meters, with a coil on the bottom is about 42".

As far as measurements including the ball at the tip, I think the difference is too insignificant to matter.

 

[2ahuman1791]

So to be clear I'm supposed to measure from the red line (top) in this photo to the yellow line bottom of detacted antenna) to get my 16 1/8" total antenna length for 1/4 155Mhz?
I should ignore the green and purple lines?
and I'll take the whip out, cut the bottom to get my total length?

 

[2ahuman1791]

Laird supplies a cutting chart for the B132S (page 2 of this pdf) --> https://www.talleycom.com/images/pdf/ANXB132S.pdf

Yes I posted that PDF earlier, but this is clarification on where to cut and measure from. I've read conflicting info on measuring and cutting the straight antenna only and refined info saying you measure the entire unscrewed base to the tip, spring and all, at least I'm waiting on clarification. I'm using a dummy diagram of my antenna which I color coded for feedback. I'm old to radios but brand new to MURS 155 green dot blue dot, antenna cutting, etc.
Sorry I'm pretty ignorant and thankful for folks teaching the new generation. My photo is waiting on moderators I think.

 

[mmckenna]

I've read conflicting info on measuring and cutting the straight antenna only and refined info saying you measure the entire unscrewed base to the tip, spring and all, at least I'm waiting on clarification.

The cut charts from the manufacturer are exactly that. It tells you how long to cut the whip. You don't measure the base or spring as part of the cut, just the total length of the whip. That's why the cutting chart lists the length as "whip length". They've already figured in the spring and base part of the equation.

 

[2ahuman1791]

The cut charts from the manufacturer are exactly that. It tells you how long to cut the whip. You don't measure the base or spring as part of the cut, just the total length of the whip. That's why the cutting chart lists the length as "whip length". They've already figured in the spring and base part of the equation.

oh ok so i m measuring from purple line to red line in my photo to get my 16 1/8" length. just the whip itself. Got it.
Thank you for this clarification on this antenna type. I appreciate it!

 

[nd5y]

oh ok so i m measuring from purple line to red line in my photo to get my 16 1/8" length.

No. You measure the whip length with the whip removed.
The whip length specified in the manufacturer's cutting chart should include the 1/2" or so of the whip that goes inside the cone with the set screw on the top end of the spring.

That is unless they provide a drawing that shows something different.

 

[2ahuman1791]

No. You measure the whip length with the whip removed.
The whip length specified in the manufacturer's cutting chart should include the 1/2" or so of the whip that goes inside the cone with the set screw on the top end of the spring.

That is unless they provide a drawing that shows something different.

Alright glad to know this. Thank you.

 

[mmckenna]

Yeah, it's total whip length when it's removed from the base. Total length of the whip in it's entirety, nothing more, nothing less.

"Nothin' but whip".

Keep in mind, that that will get you close enough to get good SWR.

In reality, actual tuning will depend on each individual installation. Amount of ground plane, nearby conductive materials, other antennas, coax cable, etc, will all add some variables. If you wanted to try and SWR meter, the cut whip length gets you close. The little bit of extra whip inside the mount will allow you to adjust it up and down a little bit to compensate for those variables.


I've installed a lot of 1/4 wave VHF whips at work, and I will occasionally test one. But every single time the cut chart gets you so close it isn't worth the time to mess with it, or blow money on test equipment if you don't already have it.

 

[2ahuman1791]

yes I understand now far better than before. I think I might have been conflating the 1/2 wave and 1/4 wave info in hind sight. sorry about that. I think my ignorance was to blame rather than others. It was the B132S antenna cutting guide that left me confused. so you see i traveled through a valley of duh moments. Thank you. Very nice folks here!

 

[mmckenna]

Thank you. Very nice folks here!

Well, there's a reason why we know this. It's because we've all struggled with it and learned it in the past. We're just doing our best to pass on that hard earned knowledge to others.

In other words, "We've already made the mistakes so you don't have to."

 

[2ahuman1791]

Well, there's a reason why we know this. It's because we've all struggled with it and learned it in the past. We're just doing our best to pass on that hard earned knowledge to others.

In other words, "We've already made the mistakes so you don't have to."

Thank you and others for the much needed help and not being too skilled for the new guy. I was nervous just to post such a seemingly dumb thing. I mean I had the PDF but that only added to my questions honestly. It measure twice, cut once and I was headed in a goofy direction.

 

[mmckenna]

It measure twice, cut once and I was headed in a goofy direction.

It's good to ask for clarification, once you cut the whip, it's hard to make it longer again.


I used to work for a guy that loved to say "I cut it three times, and it's still too short!". Which is a humorous way to say "measure twice, cut once".

 

[Ubbe]

In metric the speed of radiowaves are 300 million meters per second and millions are the same as Mega, that rimes well with mega hertz. So 300 divided by the frequency in MHz result in the length of a full wave of that frequency. Then you want 1/4 wave so divide by 4.

The metal surface of the antenna will give some delay factor to the radio wave travelling over it so multiply with .96 to get the length for a 1/4 wave antenna element. That is from the tip of the element to it's ground plane, that would be the bottom of that magnet that are placed on metal. If you drill a hole in a car roof and install without the magnet then it would instead be the distance between your red and yellow lines.

For a 155MHz antenna it is a total length of 465 mm. 18 1/4"

If having access to a SWR meter you can use a crocodile clip on top of the whip to see if the SWR went down, then the whip are already too short. Or you can probably use a paper clip, just anything of metal that would extend the whip just a little bit. But a 1/4 wave are the most forgiving type of antenna and a little SWR do not matter much for direction pattern, range and bandwidth.

/Ubbe