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weird swr reading.


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#1 WRFV510

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Posted 31 July 2020 - 12:09 PM

Ok i started tuning my Laird BB4505CR mobile antenna and i cut it too short of course.  1.7 SWR, @ 462.550 1.2 SWR @467.7250.  but @465.0000 i have 3.2SWR. how is that? 

edited

because i screwed up in CPS and forgot to set the TX frequency to 465.0000 instead of the default 403.0250Mhz



#2 kidphc

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Posted 31 July 2020 - 12:15 PM

Colinear design. Generally, if you cut it messes it up a bit. Colinear antennasn which are basically stacked dipoles. It also has a loaded coil in the base. They usually come tuned from the factory.

Generally, don't tune that design.

Which end did you clip?

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#3 marcspaz

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Posted 31 July 2020 - 01:01 PM

The Laird B4505C and it sub-models (BB4505CR and B4505CR) are non-tunable, pre-factory tuned antennas. 

 

If you recall from my Gain and SWR thread, I mentioned that some antennas are not designed to be tuned by the user.  You need to read the product manuals for tuning instructions.  In this case, you would have had to looked at relocating the antenna or checking for damage.  Tuning isn't an option for you.

 

Unfortunately, that antenna is likely toast.  Even if you get a replacement whip, the chances of getting it within spec is almost zero. The whip being 0.5mm too high or too low could cause issues.

 

I would recommend buying another antenna.



#4 berkinet

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Posted 31 July 2020 - 02:28 PM

Just for a different take on this, since the frequencies you want to transmit on are looking good, and you are never going to transmit on 465, I’d just let it go and give it a try.
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#5 WRFV510

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Posted 31 July 2020 - 04:05 PM

it comes almost pre tuned to 450MHz @13-1/8" 465.0000 you cut to 11-11/16th.  says so on the package. wish i could load pics on this forum. lol i screwed up in CPS i just noticed the  i forgot to set the TX frequency. it was at the default 403.025Mhz. So even though my display was 465.000 it was tx on 403.0250.  and thats why i cut to short. good thing i checked lowest and highest frequency im going to use.  1.7swr is good to go. i will buy new wips and tune them right and just keep theses as spares. I also cut a BB4503S too. SWR is about the same on that one too



#6 WRFV510

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Posted 31 July 2020 - 04:11 PM

i clipped the bottom like the directions said. 



#7 JohnE

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Posted 31 July 2020 - 06:56 PM

always, always cut them longer.

you can tune w/set screw as there is ~1/2" of play in there.

the big M's spectrum antennas were hard to get right the first time and had no real BW

AS(now PCTel) mosaic antennas have a lot of leeway to tune and is my preferred gain antenna 


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#8 WRFV510

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Posted 31 July 2020 - 07:12 PM

ya i did cut longer. then because i didnt know i was tx on 403.025 instead of 465.000 on the 2nd cut  the SWR was over 3.  on the 3rd cut the (recommended length) i checked lowest and highest frequencies and saw it was 1.7 and 1.2.    their measurement is slightly too short for my set up. Motorola XPR5550e.  both antennas are at the  specs listed for 465.000Mhz. I was wondering why when i tried to calibrate the SWR the needle was going lower and lower. the radio was cutting power because of the high swr. 



#9 marcspaz

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Posted 31 July 2020 - 10:14 PM

i didnt know i was tx on 403.025 instead of 465.000

 

 

I'm confused.  GMRS is no where near 403.  Its 462 and 467.



#10 WRFV510

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Posted 31 July 2020 - 10:47 PM

my radio XPR5550e is programmable from 403.025Mhz to 470Mhz.  by default when you program a channel you add the channel to the zone. then you have to set everything up manually.  Motorola CPS is doesnt do anything automatically for you. you have to remember to set your tx and rx frequencies or the one you forget will stay at 403.0250Mhz.  I some how forgot to program the RX frequency so it stayed at the default 403.0250Mhz. I set the RX to 465.0000mhz but forgot to change the TX to 465.0000Mhz. The radio only displays what you name the channel so I just see 465.0000 because i name each channel by frequency. 



#11 marcspaz

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Posted 01 August 2020 - 08:37 AM

Okay... I'm tracking now.

#12 tweiss3

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Posted 01 August 2020 - 10:56 AM

If you wanted super wide band usage, then a trim to SWR was a poor choice. Should have chosen a pretuned option. Typically trim to tune only have a bandwidth of 10MHz, where as wide band options with a coil can cover the entire UHF band (400-512) like the Larsen NMO150/450/758 which does a great job on all of UHF as well as VHF and the 700/800 band.

#13 WRFV510

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Posted 01 August 2020 - 12:04 PM

i only want GMRS. the 403.025 is just what the lowest frequency the radio can be programmed to. the band with on these are 20Mhz. they are tune able from 450-470Mhz 



#14 axorlov

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Posted 01 August 2020 - 03:01 PM

I agree with berkinet, I think you're fine with 1.7 SWR on 462MHz. Perfect is an enemy of good.

There is another set of issues when trying to improve from 1.7 to 1.1:

 

- Do you trust your meter;

- Are your measurements consistent;

- What is the setup, and how much of the effect the environment have on your measurements;

- Is your feed line choked enough for 462 MHz (this is related to the question above) - it's not trivial, btw;

And there are more and more...


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#15 WRFV510

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Posted 01 August 2020 - 10:35 PM

the meter is a MFJ 874

yes i triple checked my measurements just didnt double check my CPS tuning. 

Motorola XPR5550e 403-470Mhz 40w to a Laird RG8X NMO roof mount Mini UHF connector Laird BB4505CR https://www.theanten...s-bb4505cr-6538

feed line is RG8X with a mini uhf connector installed by The Antenna Farm. Its a Laird MB8X https://www.theanten...ogies-mb8x-1141

antenna is in center of my 17 Silverado crew cab. 






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