Antenna Grounding Question

[JoshuaHufford]

I'm planning on at some point down the road mounting a Slim Jim copper pipe antenna in a permanent location on a mast. Everything I've read recommends directly grounding the antenna and the mast via a ground wire to a ground rod for safety, which makes sense. And also the antenna can be in direct contact with a metal mast.

What I don't understand and would like someone to explain is how does this not affect reception? I obviously don't have a good understanding of how this works, but the electrical understanding I do have would make me think that any signal the antenna picks up will take the path of least resistance which would be to ground and not to the receiver. So how does this work?

Thanks!

 

[ko6jw_2]

It's magic. Seriously, this type of shunt fed j-pole has a feed point the is electrically above the ground at RF. The j portion is a quarter wave matching transformer. In theory the impedance is between zero and infinity depending on where you attach the feed line. Somewhere above the bottom of the matching section is 50 ohms. Therefore, the part of the j below the feed point is no longer a factor and can be grounded. It is important not to equate the DC ground with the RF ground.

 

[JoshuaHufford]

Thanks for explaining. So as long as the mast is attached below the coax connector I'm fine?

 

[ko6jw_2]

I'm not sure of the exact design of the Slim Jim as opposed to a common copper pipe J-pole. However, the classic design is to have a copper tee connector where the matching stub connects physically to the vertical section. The length of pipe below the tee is used to attach to the mast with some suitable clamp.

 

[JoshuaHufford]

Here is the antenna,

VHF Public Safety, MURS, Marine, & Scanner Slim Jim Antenna

There would be no way to easily mount this to a mast and keep the mast below the feed point. The picture shows using a section of PVC pipe between the mast and the antenna, although everything I've read still says it can be directly mounted to the mast.

 

[JoshuaHufford]

Interestingly this page says that for the Slim Jim, no part should be grounded and it should be insulated from its mast.

SLIM JIM ANTENNA PROJECT

 

[prcguy]

Personally I would avoid the Slim Jim or any kind of J-pole antenna and go with something else. For antenna grounding info Google search "NEC article 810" which deals specifically with antenna grounding.

 

[JoshuaHufford]

Personally I would avoid the Slim Jim or any kind of J-pole antenna and go with something else.

Can you explain why?

 

[prcguy]

Mediocre performance, odd grounding needs, etc. They are also known to light up the coax with RF, meaning the outer braid of the coax becomes a radiating part of the antenna unless you apply proper RF choking at the feed point. How much do you have to spend on an antenna?

Can you explain why?

 

[JoshuaHufford]

I can go another route sure. Almost everything I've read about the design is positive, yours is honestly the first negative.

Do you have a suggestion? This is only for the Railband. 160-162MHz

 

[JoshuaHufford]

Receive only.

 

[WA0CBW]

Because it isn't grounded for one thing. Notice the PVC pipe mast (not metal) in the picture. As prcguy said there are better choices for an antenna. Not saying it won't work but it will take some fiddling and something more than an SWR meter.
Bill

 

[JoshuaHufford]

Well, I'd be willing to spend up to $200, I want it to be a good antenna and receive as well as possible, but I also want it to last, I do not do well with heights and this will be about 30Ft. up in the air on a steep pitched metal roof so I don't want to have to get on the roof any more than I have to.

I did look at this one,

https://www.theantennafarm.com/catalog/diamond-f23h-4865

I wasn't sure how difficult this would be to tune as I don't have a SWR meter or analyzer.

 

[AK9R]

A quarter-wave ground plane antenna will have the same gain as a J-pole or "slim Jim" antenna and doesn't have the grounding issues.

https://www.arrl.org/files/file/Technology/tis/info/pdf/ab18-16.pdf

 

[Ubbe]

https://www.theantennafarm.com/catalog/diamond-f23h-4865

I wasn't sure how difficult this would be to tune as I don't have a SWR meter or analyzer.

Not difficult at all, there's a cutting chart included to cut it to exact lenght that Diamond have tested out for you.
I believe this to be an excellent antenna (the best non-professional VHF vertical?) that will not disappoint you if you mechanicly can secure a big antenna.

/Ubbe

 

[prcguy]

I've used similar antennas and this would be quite an upgrade from a J-pole type. The gain figures are a bit inflated but it should give you really good RR reception compared a typical wide band scanner antenna.

Well, I'd be willing to spend up to $200, I want it to be a good antenna and receive as well as possible, but I also want it to last, I do not do well with heights and this will be about 30Ft. up in the air on a steep pitched metal roof so I don't want to have to get on the roof any more than I have to.

I did look at this one,

https://www.theantennafarm.com/catalog/diamond-f23h-4865

I wasn't sure how difficult this would be to tune as I don't have a SWR meter or analyzer.

 

[JoshuaHufford]

How is its durability? I was looking at a similar Tram a while back but read some reviews that it got water inside it after a few years.

 

[prcguy]

I can't speak for the specific antenna you chose but I did just buy a Tram 1481 dual band 17ft long amateur antenna and put it up at a remote site just a few days ago and I was just using it a few minutes ago. It only cost $89 and some change new including shipping. I coated the base of each fiberglass section with silicone sealant so it squirted out of the chrome couplings when tightened. This antenna will not be leaking from the fiberglass junctions.

The rest of the antenna looks ok and I don't think water will get into other parts unless its squirted at high pressure upwards into the base area. I would never use one of these on a mountain top repeater site but for home use it should be ok for many years. I also paint all of my fiberglass antennas with a couple of coats of light grey to tone them down for the neighbors and to protect the fiberglass.

I went cheap on the mounting and reused an old 2" dia Directv J mast screwed into some rafters. The Tram antenna has less wind load than a Directv dish so it should be ok unless a tornado hits, which can happen at its Dallas, TX location. I believe the fiberglass will snap before the mount lets go.

How is its durability? I was looking at a similar Tram a while back but read some reviews that it got water inside it after a few years.

 

[majoco]

Read the original article for the "Slim Jim" and ignore all the other 'variations'. There is NO ground to the metalwork other than the shield of the coax.

Original Slim Jim Antenna Article by F.C. Judd - The 2BCX Slim Jim Antenna - G2BCX

 

[chief21]

If you're looking at a receive-only antenna, I've been reading good things about this one...

OmniX Scanner Base Antenna