Newbie - High SWR on KB9VBR Slim Jim

[dpslusser]

Hi Everyone,

 

I'm new to GMRS and need a bit of guidance on where to start troubleshooting my first base station setup out at my farm in the country (outside of town). I own 100 acres with a 30 acre field. No utility power at the garage currently (it's a work in progress). No other buildings around. Garage is on top of the hill in the property with 360degree view of the property. Perfect spot for a base station radio or future repeater. 

Current hardware deployed for Base Station:

• KB9VBR Slim Jim Jpole Antenna (**Edited to show range 462-467Mhz GMRS)
• 30ft Push-up Mast
• 50ft of LMR400 (had this laying around)
• BTech 50V2 Radio

Right now, everything is temporarily installed. Slim Jim clamped to PVC Pipe which is clamped to Push-up Mast. Mast is temporarily supported next to my garage. Coax strapped down to the pole which is connected to the Btech sitting on a table inside the garage door. I use a car battery to power the Btech radio. Coax literally is a straight run to the radio. No choke loops, surge suppressors, ground straps, etc..

I only setup the above for a few hours this afternoon to do some testing.

No matter what channel I Tx on, my SWR is typically 2.75-2.95. But, when running the Btech in my truck with a Midland MXTA26 mag mount antenna, the SWR is a 1.17 typically. Why such a big difference? I would of expected an even lower SWR completed isolated from the truck, and with the antenna 30ft away from the radio.

Should I be concerned about the high SWR?

[SteveShannon]

7 minutes ago, dpslusser said:

Hi Everyone,

 

I'm new to GMRS and need a bit of guidance on where to start troubleshooting my first base station setup out at my farm in the country (outside of town). I own 100 acres with a 30 acre field. No utility power at the garage currently (it's a work in progress). No other buildings around. Garage is on top of the hill in the property with 360degree view of the property. Perfect spot for a base station radio or future repeater. 

Current hardware deployed for Base Station:

• KB9VBR Slim Jim Jpole Antenna
• 30ft Push-up Mast
• 50ft of LMR400 (had this laying around)
• BTech 50V2 Radio

Right now, everything is temporarily installed. Slim Jim clamped to PVC Pipe which is clamped to Push-up Mast. Mast is temporarily supported next to my garage. Coax strapped down to the pole which is connected to the Btech sitting on a table inside the garage door. I use a car battery to power the Btech radio. Coax literally is a straight run to the radio. No choke loops, surge suppressors, ground straps, etc..

I only setup the above for a few hours this afternoon to do some testing.

No matter what channel I Tx on, my SWR is typically 2.75-2.95. But, when running the Btech in my truck with a Midland MXTA26 mag mount antenna, the SWR is a 1.17 typically. Why such a big difference? I would of expected an even lower SWR completed isolated from the truck, and with the antenna 30ft away from the radio.

Should I be concerned about the high SWR?

What frequency is your slim Jim tuned for?

[dpslusser]

1 minute ago, Sshannon said:

What frequency is your slim Jim tuned for?

462-467Mhz GMRS

[MarkInTampa]

Have you tried checking the SWR on the slim-jim without the additional coax? Just to rule out possible bad coax to the antenna.

Here's a SWR sweep of my N9TAX GMRS slim-jim, around 1.3 SWR on both sides of GMRS

[SteveShannon]

6 minutes ago, dpslusser said:

462-467Mhz GMRS

Swr shouldn’t be that high if the antenna is tuned right, but even that high doesn’t imply much loss unless you’re using cable with a lot of attenuation. LMR400 isn’t bad usually but could be damaged. 
Of course a really crappy cable will always yield a perfect SWR.

Honestly that high of SWR isn’t a real concern but you might try testing SWR with a different cable to see if it helps. 

[MarkInTampa]

Sorry, thought you were talking about the N9TAX slim-jim that has a 6ft long or so pigtail. Here's a SWR plot of my KB9VBR GMRS J-Pole. Around 1.8 SWR on the low end of GMRS and around 1.3 SWR on the high end. I stopped using it awhile back, just didn't work as well as other antenna's I have.

 

[dpslusser]

I'll connect directly to the Jpole tomorrow or Monday and get SWR readings. Thanks guys. 

When I go to a permanent mount, other than a Surge Suppressor to a ground rod. Any other suggestions for installation? Shouldn't I put in a choke to be safe?  I am a bit concerned about lighting strikes being I'm on top the hill  . 

[WRYZ926]

Make sure the antenna/mast is grounded along with using a lightening arrestor where the coax enters the building. 

You can roll up any extra coax at the antenna end which will act as a RFI choke, but it shouldn't be necessary.

[WRWE456]

I have the KB9VBR GMRS Slim Jim also. Tested at 1.3 with 50' LMR 400.

[Lscott]

1 hour ago, WRYZ926 said:

You can roll up any extra coax at the antenna end which will act as a RFI choke, but it shouldn't be necessary.

For a J-Pole type antenna a coax line choke at the base of the antenna is HIGHLY recommended.

[WRYZ926]

11 minutes ago, Lscott said:

For a J-Pole type antenna a coax line choke at the base of the antenna is HIGHLY recommended.

I did not know that since I have never used a J pole antenna. I do put chokes on all of my HF antennas though.

[WSBZ540]

Is it a slim jim, or a jpole? J pole looks like a big letter J, and the slim jim looks like a folded dipole (an extremely stretched out rectangle with a small gap on one side).

Both will benefit from 5 or 6 loops of coax under it, but I build a lot of slim jim and jpole antennas and the slim jim seems to be more picky about where the coax runs after the connector. (the jpole as well but less so) Make sure it 'exits' on the stub (short side) and doesn't cross back over it any where, or it will make your swr run a little crazy. 

[bshort]

I have the KB8VBR gmrs, 1.25 and 2m slim jims and they are all around 1.2.  I also use three ferrite beads as well.  Is your antenna close to a metal roof? 

[nokones]

16 hours ago, dpslusser said:

Hi Everyone,

 

I'm new to GMRS and need a bit of guidance on where to start troubleshooting my first base station setup out at my farm in the country (outside of town). I own 100 acres with a 30 acre field. No utility power at the garage currently (it's a work in progress). No other buildings around. Garage is on top of the hill in the property with 360degree view of the property. Perfect spot for a base station radio or future repeater. 

Current hardware deployed for Base Station:

• KB9VBR Slim Jim Jpole Antenna (**Edited to show range 462-467Mhz GMRS)
• 30ft Push-up Mast
• 50ft of LMR400 (had this laying around)
• BTech 50V2 Radio

Right now, everything is temporarily installed. Slim Jim clamped to PVC Pipe which is clamped to Push-up Mast. Mast is temporarily supported next to my garage. Coax strapped down to the pole which is connected to the Btech sitting on a table inside the garage door. I use a car battery to power the Btech radio. Coax literally is a straight run to the radio. No choke loops, surge suppressors, ground straps, etc..

I only setup the above for a few hours this afternoon to do some testing.

No matter what channel I Tx on, my SWR is typically 2.75-2.95. But, when running the Btech in my truck with a Midland MXTA26 mag mount antenna, the SWR is a 1.17 typically. Why such a big difference? I would of expected an even lower SWR completed isolated from the truck, and with the antenna 30ft away from the radio.

Should I be concerned about the high SWR?

You should put a quality antenna analyzer on your J Pole without the cable and sweep it and see what the antenna resonates at. Then try with the cable attached and sweep it. Then put the MXTA 26 at the end of the coax in place of the J Pole and sweep it. The results should indicate where your problem maybe located.

If you need to replace the cable "Do Not Buy Cheap Cable" from Amazon.

[WRYZ926]

1 hour ago, nokones said:

If you need to replace the cable "Do Not Buy Cheap Cable" from Amazon.

This is very important to remember. Get the best and correct type of coax you can afford.

[WRKG321]

That antenna is hung from the top. Is your mast non conducting? If the antenna is against a conductive mast, it can cause problems. I only ask because I have seen this before. 

[WSAM454]

27 minutes ago, WRKG321 said:

That antenna is hung from the top. Is your mast non conducting? If the antenna is against a conductive mast, it can cause problems. I only ask because I have seen this before. 

The  KB8VBR gmrs antenna  that the OP said he is using is a copper tubing type of  J-Pole style and mounts to a length of pipe or tubing.

https://www.jpole-antenna.com/shop/462-mhz-land-mobile-gmrs-j-pole-antenna/

 

If he were using a twin-lead type "slim jim" antenna, (like the N9TAX antenna )  then yes, it must be hung from the top and remain in the clear.  Not the easiest thing to do for a permanent installation.   (I am not sure why the OP used the term "slim jim" along with the KB8VBR antenna description, since the link clearly shows it is not that style of antenna!)

[SteveShannon]

7 hours ago, WSAM454 said:

(I am not sure why the OP used the term "slim jim" along with the KB8VBR antenna description, since the link clearly shows it is not that style of antenna!)

Possibly because copper tubing slim-jims are also sold on KB9VBR’s site:

https://www.jpole-antenna.com/shop/uhf-440-470-mhz-slim-jim-antenna/

What distinguishes a slim-jim is the folded dipole design, not that it’s made from flatline.