Laird FG4607 UHF GMRS Repeater Antenna

BlueDevil · Nov 25, 2017

I am having issues with my Laird FG4607 UHF Antenna that I plan on using for my GMRS Repeater.

As soon as I amount it to the roof mounting system I notice that my SWR readings go up through the roof.

It's a brand new antenna and the SWR readings are great all the way up till the point that the antenna is attached to the pole/post. I noticed that the pole/post is painted and the mounting brackets are not getting good metal to metal contact on the pole or post.

I will test some other things as well. I just haven't come across any documentation or specifications that say that this antenna needs or does not require a ground or ground plane for proper propagation.

mmckenna · Nov 26, 2017

Is the antenna mounted in such a way that the vertical radiator is completely above the mounting pole?

BlueDevil · Nov 26, 2017

Yes, The antenna is mounted as high up on the pole as possible with the antenna above the end of the pole.

BlueDevil · Nov 26, 2017

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mmckenna · Nov 26, 2017

I'd suspect an issue with the coax, or at least that's where I'd start.

cmdrwill · Nov 26, 2017

The antenna seems to be mounted correctly. I have had the antenna fail inside.

Also as mc says check the coax cable and both connectors for problems, like poor mating, at the antenna.

WQWG712 · Nov 28, 2017

No offense but why are you using that coax?

You should use LMR400 Minimum for a repeater setup.

Your Coax can affect your SWR's. Also, That Laird antenna specs: 460-470 MHz, 7 dBd/ 9.15 dBi Outdoor Fiberglass Omni base Station Antenna with N-Female Connector.

Like on CB, Your Coax Length affects your SWR!

Your problem is solved, Get better Coax... LMR400 or 7/8 Hardline for 0 loss!

RFI-EMI-GUY · Nov 29, 2017

WQWG712 said:
No offense but why are you using that coax?

You should use LMR400 Minimum for a repeater setup.

Your Coax can affect your SWR's. Also, That Laird antenna specs: 460-470 MHz, 7 dBd/ 9.15 dBi Outdoor Fiberglass Omni base Station Antenna with N-Female Connector.

Like on CB, Your Coax Length affects your SWR!

Your problem is solved, Get better Coax... LMR400 or 7/8 Hardline for 0 loss!

Actually, using lossy coax like RG58 improves the measured VSWR when tested at the radio and of the cable, because the reflected power is dissipated as loss.

Always get the best coax you can afford, you don't always need the highest gain antenna.

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mmckenna · Nov 29, 2017

WQWG712 said:
No offense but why are you using that coax?

You should use LMR400 Minimum for a repeater setup.

LMR series cables can be problematic with repeater setups. Dissimilar metals in the cable, two frequencies, add a bit of moisture and intermod issues will happen.

Heliax should be used.

WQWG712 said:
Like on CB, Your Coax Length affects your SWR!

Specific lengths of coax can hide SWR issues. Ideally you want to use the minimum amount of cable necessary to get between the equipment and the antenna. No more, no less. With a proper antenna, there's no need to hide SWR issues. The old CB wives tale about using specific lengths of coax is misguided at best. Usually it doesn't take into account velocity factor of the cable.

WQWG712 said:
Your problem is solved, Get better Coax... LMR400 or 7/8 Hardline for 0 loss!

No coax has "0 loss". All feed lines will have some amount of loss. No on the LMR. Yes on Heliax, but the size cable will depend on the length of the cable run, how much loss is acceptable for system performance and how much of a budget he has.

BlueDevil · Dec 2, 2017

I have tested everything on it's own and it all checks out up until I mount the antenna on the pole. Then my SWR readings go through the roof!

WQWG712 said:
No offense but why are you using that coax?

You should use LMR400 Minimum for a repeater setup.

Your Coax can affect your SWR's. Also, That Laird antenna specs: 460-470 MHz, 7 dBd/ 9.15 dBi Outdoor Fiberglass Omni base Station Antenna with N-Female Connector.

Like on CB, Your Coax Length affects your SWR!

Your problem is solved, Get better Coax... LMR400 or 7/8 Hardline for 0 loss!

I elected to stay away from the LMR400 because of the dissimilar metals used in that coax. I would have liked to used 7/8 Heliax Hardline for the feedline but I didn't really want to put that much money into this project. Instead I went with true MIL Spec RG-214 Coax. I figured that this would be adequate enough since the feedline is only 25ft from repeater to antenna.

WQWG712 · Dec 2, 2017

mmckenna said:
LMR series cables can be problematic with repeater setups. Dissimilar metals in the cable, two frequencies, add a bit of moisture and intermod issues will happen.

Heliax should be used.

Specific lengths of coax can hide SWR issues. Ideally you want to use the minimum amount of cable necessary to get between the equipment and the antenna. No more, no less. With a proper antenna, there's no need to hide SWR issues. The old CB wives tale about using specific lengths of coax is misguided at best. Usually it doesn't take into account velocity factor of the cable.

No coax has "0 loss". All feed lines will have some amount of loss. No on the LMR. Yes on Heliax, but the size cable will depend on the length of the cable run, how much loss is acceptable for system performance and how much of a budget he has.

No argument there

RFI-EMI-GUY · Dec 2, 2017

BlueDevil said:
I have tested everything on it's own and it all checks out up until I mount the antenna on the pole. Then my SWR readings go through the roof!

I elected to stay away from the LMR400 because of the dissimilar metals used in that coax. I would have liked to used 7/8 Heliax Hardline for the feedline but I didn't really want to put that much money into this project. Instead I went with true MIL Spec RG-214 Coax. I figured that this would be adequate enough since the feedline is only 25ft from repeater to antenna.

Hmm, seems you are doing the right things. Have you fixed the bonding of the clamp through the paint? Is the mounting frame grounded for lightning protection? Do you have a lightning protector? What model and where is it located?

This is interesting:

"The Laird FG4607 omnidirectional base station antenna incorporates a collinear design that is enclosed in high density fiberglass, which is covered with a protective ultraviolet inhibiting coating. The radiating elements are carefully phased to provide maximum gain in the horizontal plane. The mounting sleeves are tuned to eliminate RF currents from the transmission line, resulting in a “cold” sleeve that allows for greater freedom in mounting. The antenna’s high quality and well-focused beam provides the best efficiency with highest gain."

- Many the traditional collinear antennas use a set of radial decoupling tabs at the bottom of the antenna radome and attached to the mounting sleeve that are like a short ground plane adjusted 30 degrees or so downward. The Laird design talks of the mounting sleeve serving this purpose as some sort of choke element. Is there anything in the way you mounted the antenna that differs from the installation instruction?

It seems like a nice antenna. Have you called Laird customer service?

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BlueDevil · Dec 2, 2017

I have used this antenna in other systems with good success. Although those systems have had a better ground plane to work with. I am getting a different metal post to mount the antenna to, one that has no paint to deal with.

I do have a lightning protector that I plan on using however I have left it out of the system while I am trying to troubleshoot my antenna issues.

I am also going to borrow a different antenna, one with some radials to help provide a ground plane and see how my SWR readings differ.

BlueDevil · Dec 2, 2017

Here is a picture from a previous project/installation

RFI-EMI-GUY · Dec 3, 2017

BlueDevil said:
Here is a picture from a previous project/installation

Broken link.....

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jeepsandradios · Dec 3, 2017

I have used that antenna on a piece of PVC pipe with our SAR TAC repeater with no issues. I had a 25' piece of 213 duck taped to the mast. We were only running 20 watts but it worked for the event we had and tested fine on my Bird watt Meter.

Does your 213 go directly to the antenna with an adapter or the correct N for it ? I have seen some N adapters with a bad center pin cause issues.

BlueDevil · Dec 3, 2017

kb2ztx said:
I have used that antenna on a piece of PVC pipe with our SAR TAC repeater with no issues. I had a 25' piece of 213 duck taped to the mast. We were only running 20 watts but it worked for the event we had and tested fine on my Bird watt Meter.

Does your 213 go directly to the antenna with an adapter or the correct N for it ? I have seen some N adapters with a bad center pin cause issues.

The RG-214 coax that I am using has Type N Male connectors on each end that attach directly to the duplexer and antenna. No adapters are being used. I have tested everything individually and the issue seems to be the antenna mounted on the pole. I have connected my SWR meter to the antenna with a short jumper and my readings are sky high.

RFI-EMI-GUY · Dec 3, 2017

BlueDevil said:
The RG-214 coax that I am using has Type N Male connectors on each end that attach directly to the duplexer and antenna. No adapters are being used. I have tested everything individually and the issue seems to be the antenna mounted on the pole. I have connected my SWR meter to the antenna with a short jumper and my readings are sky high.

Have you tried connecting your VSWR meter at the radio end of the coax to see what the VSWR is? There might be some interaction when measuring directly at the base of the antenna. Also, is your jumper working properly? Test cables take a lot of abuse.

I would be checking the N connectors on all of the cables to be sure the center pin is flush with the end of the connector. If it sticks out too far it will splay and possible damage the female connector on the antenna or SWR meter. If it does not come out far enough it will be an intermittent connection if at all.

What kind of VSWR meter are you using? Are there any connectors in the circuit that are not N connectors? PL259's are notorious problems at UHF, BNC connectors sometimes are poor at mating.

BlueDevil · Dec 3, 2017

RFI-EMI-GUY said:
Broken link.....

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I tried inserting the picture using the "Insert Image" tool and filling in the hyperlink. I will try a couple different ways.

RFI-EMI-GUY · Dec 3, 2017

Hmm; the bracket on the earlier install is an offset bracket, 90 degrees, adds no length to the skirt. I wonder....

still, check all those connectors etc per my above....

Laird FG4607 UHF GMRS Repeater Antenna

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