Thoughts on the Slim Jim VHF Antenna?

[emsflyer84]

Hey guys, I’m on a quest for increased range with my scanner setup at home. I’ve tried a homemade PVC pipe contraption with a copper element, a Centerfire folded dipole and a ground plane kit with a Larsen tri-band antenna. All worked relatively well. I’ve increased my height as much as I can but there are still frequencies that come and go depending on the weather.

I’m looking for input or thoughts on this:

VHF Public Safety, MURS, Marine, & Scanner Slim Jim Antenna - KB9VBR Antennas

Our extremely popular Slim Jim Antenna is now available for the VHF 150 - 160 MHz band. This means our Slim Jm is optimized for VHF Public Safety, MURS, Marine band, and Business frequencies. The Slim Jim is an end fed folded dipole antenna that gives you approximately 5.5db of gain. Plus the...

www.jpole-antenna.com

His antennas have great reviews for amateur radio use. I’m wondering if just the larger size of this one will help with reception. You’ll have to excuse me, I have very little knowledge of this stuff.

Right now I’m using the Centerfire folded dipole which seems to have the best range of any setup I’ve tried so far, but the signal can be very noisy, and sometimes transmission that are close sound much farther away then they are. It’s strange.

Anyway, any thoughts would be great. Thanks!

 

[prcguy]

Great antenna, just subtract 5.5dB from the advertised gain and that's what you will end up with if you buy one.

Hey guys, I’m on a quest for increased range with my scanner setup at home. I’ve tried a homemade PVC pipe contraption with a copper element, a Centerfire folded dipole and a ground plane kit with a Larsen tri-band antenna. All worked relatively well. I’ve increased my height as much as I can but there are still frequencies that come and go depending on the weather.

I’m looking for input or thoughts on this:

VHF Public Safety, MURS, Marine, & Scanner Slim Jim Antenna - KB9VBR Antennas

Our extremely popular Slim Jim Antenna is now available for the VHF 150 - 160 MHz band. This means our Slim Jm is optimized for VHF Public Safety, MURS, Marine band, and Business frequencies. The Slim Jim is an end fed folded dipole antenna that gives you approximately 5.5db of gain. Plus the...

www.jpole-antenna.com

His antennas have great reviews for amateur radio use. I’m wondering if just the larger size of this one will help with reception. You’ll have to excuse me, I have very little knowledge of this stuff.

Right now I’m using the Centerfire folded dipole which seems to have the best range of any setup I’ve tried so far, but the signal can be very noisy, and sometimes transmission that are close sound much farther away then they are. It’s strange.

Anyway, any thoughts would be great. Thanks!

 

[emsflyer84]

Great antenna, just subtract 5.5dB from the advertised gain and that's what you will end up with if you buy one.

Thanks for the reply. I’ve seen others say the same about the gain. Why is that? Thanks.

 

[nd5y]

It's a half wave antenna just like a j-pole or dipole. It has 0 dBd gain in free space with no feedline. Probably less than that in real life because of feedline radiation.

If you want a good single band scanner antenna you would be better off with a dipole or ground plane.

 

[prcguy]

A J-pole is a half wave end fed antenna with similar gain to a 1/2 wave dipole. The little "d" suffix in dBd gain means gain compared to a 1/2 wave dipole, so a J-pole would have 0dBd gain. I think KB9VBR has his head screwed on sideways because he is claiming his folded dipole version has an additional 2.5dB gain because it has a second 1/2 wave element. And he is sort of using dBi gain and starting with a baseline of 2.5dB gain for a regular J-pole, but the difference between dBi gain and dBd gain is 2.14dB. Where is he getting the extra .36dB of gain? I think somewhere below his belt and that is beyond complete BS.

If you take an additional 1/2 wave element and position it above or below the first element at just the right spacing and split the power to both 1/2 wave elements as in a two element dipole array you can get close to 3dB gain over the single 1/2 wave element. If you add a second 1/2 wave element by stacking it on top of the first and provide a 1/4 wave phase shift between elements you can get a little more gain, not 3dB because the spacing is not optimum, but maybe a couple of dB. If you look at the old Cushcraft Ringo Ranger its two half wave elements with a 1/4 wavelength phasing stub between them and it has a little more gain than the older single element 1/2 wave Ringo.

So KB9VBR folds his element over so its like a folded dipole. Its not an extra 1/2 wave element, its just a different style single 1/2 wave element that adds no gain. It does add band width similar to making the element very fat, but no extra gain. None. Nada. But it does qualify him for extra bonus point BS.

Thanks for the reply. I’ve seen others say the same about the gain. Why is that? Thanks.

 

[emsflyer84]

A J-pole is a half wave end fed antenna with similar gain to a 1/2 wave dipole. The little "d" suffix in dBd gain means gain compared to a 1/2 wave dipole, so a J-pole would have 0dBd gain. I think KB9VBR has his head screwed on sideways because he is claiming his folded dipole version has an additional 2.5dB gain because it has a second 1/2 wave element. And he is sort of using dBi gain and starting with a baseline of 2.5dB gain for a regular J-pole, but the difference between dBi gain and dBd gain is 2.14dB. Where is he getting the extra .36dB of gain? I think somewhere below his belt and that is beyond complete BS.

If you take an additional 1/2 wave element and position it above or below the first element at just the right spacing and split the power to both 1/2 wave elements as in a two element dipole array you can get close to 3dB gain over the single 1/2 wave element. If you add a second 1/2 wave element by stacking it on top of the first and provide a 1/4 wave phase shift between elements you can get a little more gain, not 3dB because the spacing is not optimum, but maybe a couple of dB. If you look at the old Cushcraft Ringo Ranger its two half wave elements with a 1/4 wavelength phasing stub between them and it has a little more gain than the older single element 1/2 wave Ringo.

So KB9VBR folds his element over so its like a folded dipole. Its not an extra 1/2 wave element, its just a different style single 1/2 wave element that adds no gain. It does add band width similar to making the element very fat, but no extra gain. None. Nada. But it does qualify him for extra bonus point BS.

Quite the explanation. Thanks! I actually really like the Centerfire folded dipole I’m using now, I just wish I could figure out why it seems to be inconsistent in the clarity of the received transmissions. Sometimes it works awesome and the signals are crystal clear and reception is great, sometimes the same transmitter sites go in and out and range seems to be reduced. Maybe atmospheric forces just have more effect then I thought.

Maybe I’ll try the Larsen tri-band with Laird ground plane at the increased height the dipole is at now and see if I get a cleaner signal.

Probably I’ll need to increase height more at this point for better reception, regardless of antenna....

 

[prcguy]

At VHF/UHF and for local reception, atmospheric changes like rain, fog, hot, cold, etc, have virtually no effect on propagation or reception. If you are listening to something that is non line of site and bouncing off distant hills then that propagation can change during the year with rain soaking the soil or drying out or vegetation growing and dying, etc.

If you have line of sight signals coming and going then I would suspect local interference and possibly from inside your own home from computers, switching power supplies, LED lights and so on.

Quite the explanation. Thanks! I actually really like the Centerfire folded dipole I’m using now, I just wish I could figure out why it seems to be inconsistent in the clarity of the received transmissions. Sometimes it works awesome and the signals are crystal clear and reception is great, sometimes the same transmitter sites go in and out and range seems to be reduced. Maybe atmospheric forces just have more effect then I thought.

Maybe I’ll try the Larsen tri-band with Laird ground plane at the increased height the dipole is at now and see if I get a cleaner signal.

Probably I’ll need to increase height more at this point for better reception, regardless of antenna....

 

[emsflyer84]

At VHF/UHF and for local reception, atmospheric changes like rain, fog, hot, cold, etc, have virtually no effect on propagation or reception. If you are listening to something that is non line of site and bouncing off distant hills then that propagation can change during the year with rain soaking the soil or drying out or vegetation growing and dying, etc.

If you have line of sight signals coming and going then I would suspect local interference and possibly from inside your own home from computers, switching power supplies, LED lights and so on.

Thanks. The issues I have seem to change daily or just randomly. For example the minute I raised my dipole a few feet, some of the weaker signals started coming in better right away. Then a day later they weren’t coming in again. Then randomly the same stations I could hear clear as day... for a day or so.... it’s very odd. But I get the same issue with every antenna setup I’ve tried.

 

[GMB951]

Had one for years would rate it with the best for VHF,have mine at 30 feet

 

[prcguy]

I would recommend using a receiver with a spectral display or an SDR type that will feed your computer monitor with a spectrum view. Then you can see what else is going on when these signals fade away . It could be they are still there at the same level but interference is covering them up.

Thanks. The issues I have seem to change daily or just randomly. For example the minute I raised my dipole a few feet, some of the weaker signals started coming in better right away. Then a day later they weren’t coming in again. Then randomly the same stations I could hear clear as day... for a day or so.... it’s very odd. But I get the same issue with every antenna setup I’ve tried.

 

[emsflyer84]

I would recommend using a receiver with a spectral display or an SDR type that will feed your computer monitor with a spectrum view. Then you can see what else is going on when these signals fade away . It could be they are still there at the same level but interference is covering them up.

Thanks, I’ll look into it. Sometimes it does almost seem like an issue in my house or with my cables or something. A single transmission will start out crystal clear and then get static-y and fade to nothing, then come back strong again. All in a single 10 second transmission. This doesn’t always happen, but randomly. And on freq’s that typically come in strong.

 

[Ubbe]

Anyway, any thoughts would be great. Thanks!

What scanner or receiver are you using?
What are the model name and frequency band of your current dipole antenna?
What frequencies do you monitor?
What coax do you use and do you have it decoupled from the antenna by a balun of some sort?
Do you know if you have line of sight to that tower that has that 10 sec fading?

I have no line of sight to any transmitter and that kind of fading signals occur when the trees sway when it's windy. When not having line of sight the weather has a big impact on my reception. One day the squelch hardly opens and the next day the signal are crystal clear. But that stays fairly constant for several hours.

/Ubbe

 

[emsflyer84]

I’m using a Whistler TRX-2

Antenna is a Centerfire folded Dipole tuned to 150-165: Folded Dipole Scanner Antennas

The antenna has a 300 to 75 ohm transformer built in.

I’m using 50’ RG6 from the antenna and have another 10’ section to the scanner.

That’s about it on the setup.

It’s funny you mentioned wind. When the signal drops out that’s the first thing I thought of. I don’t know why but it almost sounds like a wind gust. The static gets louder then softer as the signal comes in and out. Like a fist of wind.

I don’t have line of site either to any tower either. There is one particular frequency that I scan, the transmitter is about 25 miles away through some small mountains. It’s digital VHF high, 154.xxx. Randomly the signal comes in crystal clear for several hours or a few days in a row. Then it drops out again for a long time. I can still hear the transmissions but can’t make them out due to digital noise. It just seems like some days overall reception is great, and other times it’s not so good.

 

[nd5y]

Randomly the signal comes in crystal clear for several hours or a few days in a row. Then it drops out again for a long time. I can still hear the transmissions but can’t make them out due to digital noise. It just seems like some days overall reception is great, and other times it’s not so good.

That could be normal for your location. If possible try moving the antenna up and down the mast a at least 1/4 wavelength (that's about 18" on VHF high band) and rotating it 90 degrees around the mast and see if anything changes.

 

[emsflyer84]

I’ll give that a shot. Not that your saying it, it almost seems like reception was slightly better when the antenna was LOWER on the mast. I just raised it a bit thinking any height would help. Maybe I’ll rotate it 90 first and see, then play with the height. Thanks.

 

[emsflyer84]

One piece of the equation I left out because I just thought of it. The dipole is sitting literally directly above a Tempest wireless weather station mounted on the same mast. The lower loop of the dipole is only inches above the top of the weather station. This is a wireless system that sends signals to a unit inside the house. I wonder if this could be contributing to the inconsistent performance, interference issues, etc...

 

[JoshuaHufford]

I bought one of his Slim Jim antennas tuned for 160-164MHz tuned to the railband, I thought about making one myself but I dont' have the equipment to tune it properly so I just bought one from him. I have to say it works quite well, and I had no complaints at all, however I doubt it would work much better if at all than what you have already tried. I ended up replacing it with a Diamond F23H because I wanted something with more gain, however when I did there was an increase in performance, but not by much. My noise floor also increased with the Diamond antenna compared to the Slim Jim, and I have heard other users report that this design works well in areas with a high RF noise floor.

As others suggested your problems sounds more like a local interference problem not an antenna problem.

You might also want to check your coax cable, could be a bad connection somewhere or water intrusion. Were all of the connections properly waterproofed when it was installed? How old is the coax?

 

[nd5y]

The dipole is sitting literally directly above a Tempest wireless weather station mounted on the same mast. The lower loop of the dipole is only inches above the top of the weather station. This is a wireless system that sends signals to a unit inside the house. I wonder if this could be contributing to the inconsistent performance, interference issues, etc...

Maybe. The transmitter could desense your scanner and the receiver & other circuitry could generate noise but that would not change over hours or days.

I had a remote thermometer that operated on 433.92 MHz. It transmitted about once a minute and didn't cause many problems, probably because it was about 50' outside but the receiver made a wideband pulsing noise about 2 or 3 times a second all over UHF.

 

[emsflyer84]

I bought one of his Slim Jim antennas tuned for 160-164MHz tuned to the railband, I thought about making one myself but I dont' have the equipment to tune it properly so I just bought one from him. I have to say it works quite well, and I had no complaints at all, however I doubt it would work much better if at all than what you have already tried. I ended up replacing it with a Diamond F23H because I wanted something with more gain, however when I did there was an increase in performance, but not by much. My noise floor also increased with the Diamond antenna compared to the Slim Jim, and I have heard other users report that this design works well in areas with a high RF noise floor.

As others suggested your problems sounds more like a local interference problem not an antenna problem.

You might also want to check your coax cable, could be a bad connection somewhere or water intrusion. Were all of the connections properly waterproofed when it was installed? How old is the coax?

My coax is all basically new RG6 and the connections are waterproof connections but I suppose I can double check them. I didn’t do any additional waterproofing other then using cable with “waterproof” connectors at each end.

What are the symptoms of water in the line? Thanks

 

[lu81fitter]

Water inside the coax can cause corrosion. It will play havoc with signal reception. Check out your waterproofing and make sure its up to snuff.
If you are only monitoring VHF-HI, try using just an 18" whip on your ground plane kit. When I change mine out, it makes a marginal difference, but it may be just enough for you.