gcopter1
Member
The KB9VBR the OP asked about.
Aviation J-Pole Antenna
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Ubbe
Member
They actually say 2% off frequency as the bandwidth for 1,5:1 VSWR which is a 4% bandwidth and it's +/-2,5MHz off the center, 124,5MHz-129,5MHz.
I installed that type of antenna for a simplex 75MHz radio on a ship and it seemed to work just as good as a standard 1/4 wave GP. One benefit are that it's DC grounded, so no build up of static electricity.
p.s. They actually say 2% of frequency and probably ment 2% of the frequency but it's a fixed frequency antenna that cannot be cut. Anyhow they says it covers 450-470 and that's +/-10Mhz and are a 4% bandwidth of the frequency.
/Ubbe
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The basic model number GP-201 comes in versions from 30MHz through 512MHz, the catalog page happens to be the UHF version. On the lower frequency models the customer cuts it to frequency.
Ubbe
Member
I have searched their whole catalog and they have nothing to offer below 138MHz.
The only DP-201 model they have are that 400MHz version. They say it is field adjustable within 450-470MHz so that 2% bandwidth are probably correct, 8MHz. The 2% are at 125MHz a 2,5MHz bandwidth and doesn't cover the whole airband of 19MHz. That folded single element antenna seems to be more narrowbanded, that is a good thing in most cases in professional use as it doesn't receive out of band frequencies as good as a non folded type.
When I measure a standard 1/4 wave GP antenna @155MHz it has a 7% bandwidth, 10MHz, at 1,5:1 VSWR and 12%, 18MHz at 2,0:1
A 2,0:1 VSWR are a 10% of the power reflected back and also the loss of signal and are usually what I, and many others, use as the bandwidth for antennas.
I would say that the less effective an antenna are the better SWR and wider bandwidth it has. A dummy load having the best SWR and bandwidth but the worst possible performance as an antenna. There where some amateur shortwave antennas where they inserted some resistors in the antenna to help the SWR and widen the bandwidth.
/Ubbe
WX9RLT
Top Dawg
Alright, I wish I had a better post then the one I am about to post. So I do apologize. I need some advice!!!
We are in need of some advice. Hopefully someone out there is able to respond and be able to help us.
So first, let me describe the problem.
Since the installation, we hardly hear anything at all.
But yet on the scanners with the default antennas (antenna that came with scanner) , I am hearing a lot.
The J-pole antenna was supposed to drastically improve the reception of the airbands.
But instead it did the complete opposite.
So our question is: What do you suggest to get this to work and improve the reception?
(Yes we know outside and height plays a huge role)
Any suggestions are greatly appreciated!
We are in need of some advice. Hopefully someone out there is able to respond and be able to help us.
So first, let me describe the problem.
- We are trying to listen to the aviation bands. 118.000-137.000
- We bought a J-Pole antenna from ~ Aviation (Airband) 118 – 128 MHz J-Pole Antenna | KB9VBR J-Pole Antennas
- We are using LMR-400 coax (6 foot) ~ https://www.timesmicrowave.com/DataSheets/CableProducts/LMR-400.pdf
- And are using the pl-259 to bnc adapter, to connect the coax to the scanner.
Since the installation, we hardly hear anything at all.
But yet on the scanners with the default antennas (antenna that came with scanner) , I am hearing a lot.
The J-pole antenna was supposed to drastically improve the reception of the airbands.
But instead it did the complete opposite.
So our question is: What do you suggest to get this to work and improve the reception?
(Yes we know outside and height plays a huge role)
Any suggestions are greatly appreciated!
ATCTech
Active Member
- Joined
- Aug 13, 2002
- Messages
- 1,857
Check the obvious first.
Use a proper Ohm meter and check end to end on the coax at the PL-259s (while NOT connected to the antenna or the radio during these tests) for continuity of both the center conductor and the shield. Don't use the basic "beep" test on the meter, set it to the lowest resistance scale it has. Anything more than tenths or even hundredths of ohms of resistance end to end over 6' is suspicions. Then check across the coax from center conductor to shield. Anything reading other than wide open (using a very high resistance scale on the meter) then the coax connection(s) are suspect. If the cable checks good repeat the tests with the BNC adapters connected.
Not knowing the source of your adapters/connectors it wouldn't be unheard of for the center pins of the BNCs to not make a good connection to the radio or the PL-259s. Brand-name connectors (Andrew, for example) are made to very high tolerance. The $2 version, not so much.
Last, if the coax and adapters check good, can you adapt one of your working antennas to the coax and see if it receives ok? If so, suspect the J-pole.
By the way, I looked at the link for the the antenna you bought. Why would anybody advertise a VHF air band antenna that only covers the bottom half of the band unless the user only wanted limited frequency range? 118-128 MHz - odd. We used some very narrow bandwidth antennas in the VHF range for a couple of very specific purposes, but that was really, really rare.
Cheers!
Use a proper Ohm meter and check end to end on the coax at the PL-259s (while NOT connected to the antenna or the radio during these tests) for continuity of both the center conductor and the shield. Don't use the basic "beep" test on the meter, set it to the lowest resistance scale it has. Anything more than tenths or even hundredths of ohms of resistance end to end over 6' is suspicions. Then check across the coax from center conductor to shield. Anything reading other than wide open (using a very high resistance scale on the meter) then the coax connection(s) are suspect. If the cable checks good repeat the tests with the BNC adapters connected.
Not knowing the source of your adapters/connectors it wouldn't be unheard of for the center pins of the BNCs to not make a good connection to the radio or the PL-259s. Brand-name connectors (Andrew, for example) are made to very high tolerance. The $2 version, not so much.
Last, if the coax and adapters check good, can you adapt one of your working antennas to the coax and see if it receives ok? If so, suspect the J-pole.
By the way, I looked at the link for the the antenna you bought. Why would anybody advertise a VHF air band antenna that only covers the bottom half of the band unless the user only wanted limited frequency range? 118-128 MHz - odd. We used some very narrow bandwidth antennas in the VHF range for a couple of very specific purposes, but that was really, really rare.
Cheers!
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Only 6ft of coax, are you using the antenna indoors? Near any computer equipment or LED flood lights? I am not a fan of the J-pole but it should work much better than a stock antenna. As mentioned by ATCTech check the obvious but it would also be good if you could borrow an antenna analyzer and see if the antenna with coax attached shows some resonance in the mid air band.
With a drastic difference in reception between antennas I suspect something is broken rather than the new antenna is working poorly.
With a drastic difference in reception between antennas I suspect something is broken rather than the new antenna is working poorly.
Ubbe
Member
If that's true about the 6ft coax then the antenna could be too close to other objects and throw the whole antenna off.
Who put the connectors on the coax? Maybe it wasn't done properly and you now have a short circuit or no connection.
Best would be if you have an airport nearby so you can monitor a continous transmitting info frequency. If you can hear that with the scanner antenna, and open squelch fully, then connect your j-pole and and loosen and tighten connectors and jiggle the coax and tug on everything to check if you have some intermittent connection and move the antenna around to see if reception improves.
One thing are that some people say are that the lowest part of a j-pole, where the U are, are "cold" and can be clamped to ground and others say it has to be isolated from ground to work properly. One thing are for sure and that is that it is probably the most sensitive antenna to interfering close object there is, you have to use it in free space and not lean it agains a wall.
If you are using the SDS200, then try the invert filter. Also remember that the antenna are a balanced type and your coax are unbalanced. So when you finally get the antenna working you could at least curl the coax in 4-5 turns at a 5 inch diameter at the base of the antenna.
/Ubbe
Who put the connectors on the coax? Maybe it wasn't done properly and you now have a short circuit or no connection.
Best would be if you have an airport nearby so you can monitor a continous transmitting info frequency. If you can hear that with the scanner antenna, and open squelch fully, then connect your j-pole and and loosen and tighten connectors and jiggle the coax and tug on everything to check if you have some intermittent connection and move the antenna around to see if reception improves.
One thing are that some people say are that the lowest part of a j-pole, where the U are, are "cold" and can be clamped to ground and others say it has to be isolated from ground to work properly. One thing are for sure and that is that it is probably the most sensitive antenna to interfering close object there is, you have to use it in free space and not lean it agains a wall.
If you are using the SDS200, then try the invert filter. Also remember that the antenna are a balanced type and your coax are unbalanced. So when you finally get the antenna working you could at least curl the coax in 4-5 turns at a 5 inch diameter at the base of the antenna.
/Ubbe
Ubbe
Member
It turns out that a j-pole antenna needs to be isolated from a metal mast and/or decoupled from it, as well as the coax.
Mast Mountable J-Pole Antenna
A mast decoupling stub and other mechanical improvements mitigate structure effects and realize a mast mountable j-pole antenna.
www.hamradio.me
Slim Jim vs. Traditional J-pole Antenna
Real measurements of a Slim Jim antenna and traditional j-pole antenna reveal the truth about their respective gain and pattern.
www.hamradio.me
/Ubbe
Merovingian
Member
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- Sep 17, 2006
- Messages
- 206
I had recently been looking at getting a J-pole for air band reception until I read your post about this antenna. I did some searching and it seems that it has been discontinued, the lower frequency versions. . . It does seem to be a simple enough antenna to build. I have been considering maybe building this antenna since I can't find one ready made. Does this antenna have a specific name? I would like to try to find some plans to build this for the 127MHz frequency but I need specific dimensions for the ground plane and the vertical element, as well as the best way to construct it.
I think a ground plane with a grounded hairpin loop vertical element would probably cover the entire VHF air band and would be easy to make. Its just like a regular ground plane except the vertical element makes a U turn and heads back down to the feed point where its grounded. Here is a commercial version that's rated at 2% frequency band width, so if you use 127MHz as the center of the band it should cover 114.3 to 139.7MHz.
DB201-P
1-port omni ground plane antenna, 450–470 MHz, 360° HPBW, fixed electrical tilt. Bandwidth: 2% of frequency.www.commscope.com
The commercial version uses thicker elements than a home made job using maybe house wire or brazing rods, but I think it would work fine over the 118 to 136MHz range compared to a J-pole or simple ground plane. So who wants to make one and report on how it works?
ka3jjz
Wiki Admin Emeritus
The antenna's name is the J pole - that's not a trade name. Here's a website with the dimensions and calculator from Buck Rogers K4ABT (yes, that's his name, for you scifi fans...)
Mike
Mike
Merovingian
Member
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Sorry, I was referring to Prcguy's post about the vertical antenna that is bent down and grounded at the end.
ChrisABQ
...
I bought a J-Pole from this same gentleman tuned for public safety (155 mhz) and receive tons of traffic. I'm able to receive traffic from 100 miles away (Albuquerque to Crownpoint, NM) with 3 bars. I can receive the airport 15 miles away on all frequencies (120, 250 & 350 mhz) all ground and air traffic with 3 to 5 bars). I can receive mil air frequencies in the area along with National Forest services and UHF. The 700 & 800 bands are the only things it's not good at. These are MY results. From where I'm sitting, it's a great antenna. Make sure all your connections are good and solid, no broken solder points.
Ubbe
Member
That's all well and dandy, but for all I know you could be living on a hill top and would receive all that with a paper clip as an antenna.
Can you compare it to a previous external antenna you have used, and not connected to a RG58 and with the new j-pole you use LMR400.
/Ubbe
I've found with just simple experimentation that larger antennas that are designed to be used outdoors when used indoors may perform worse than a smaller antenna such as a 1/4 wave whip on a cookie sheet or even a rubber duck. My thought is the larger antenna is more likely to be affected by home wiring or other RF noise sources, but I'm no expert.
I have a SlimJim antenna tuned to 160-164 MHz made by KB9VBR and I'm quite impressed with it. My main interest in scanning is the railband but I also listed to a lot of other stuff as well. I first had it mounted as a temporary install on my home and it received quite well. The permanent install for this antenna was to be at my Father In Laws shop which is a better receive site than I have here at home, but I instead decided to go with another antenna because I read quite a few posts on this site about how the J-Pole and Slim Jim antennas were not a good choice.
I decided to go with this antenna instead, and I had it installed in the same exact location here at home for about a month before I moved it to it's now permanent location at the shop.
And I have to say it was probably only a small improvement over the Slim Jim, maybe 20% better, and the noise floor is higher. A quiet noise floor is a feature highlighted by the seller of the J-Pole/Slim Jim antennas. Keep in mind this is only a subjective observation, not any real test data. I'm basing this from defect detectors that I could pick up and how far out I could pick up train crews as they traveled.
I only have experience with a J-Pole type antenna with this one, but it was a positive one. I had no problems picking up my local PD and other services which are mostly all in the 150-155MHz range with it. I would say if you don't need a wide bandwidth, and you can install it properly in a good free open space and use a choke balun on the feed it can be a good antenna choice.
I now keep the antenna in my camper and use it when camping, it it obviously quite rugged so I don't have to worry about anything breaking it while traveling. I have tried it out indoors a few times when testing radios etc. and found that indoor performance, even when next to a window is quite poor.
Ubbe
Member
The Diamond F23 has a high gain figure in VHF. It is crucial to check that it doesn't overload a scanner that will make it receive worse with increased noise. Using FM trap filters with that antenna might be needed. In US you have some strong pager transmitters in the 155MHz band so getting a filter for that from PARElectronics might also be necessary. My F23 are a huge improvement over a 5/8 GP in VHF. It actually receives well in many different frequency bands and do good service as a multipurpose scanner antenna at my home.
/Ubbe
/Ubbe
Just got back from a trip to Cape Cod. Mounted my j-pole on our deck overlooking the bay. Reception on all bands was good,but aviation was tremendous! Nothing fancy but a little old copper pipe j-pole!
Yes I am quite well aware of that. My setup is about 30 Ft. of LMR-400 coax from the antenna to a 15db gain preamp, then that feeds an 8 port power divider. I'm currently feeding 4 Motorola radius receivers, 3 are monitoring data, the 4th is an audio feed. The radio that has the audio feed also has a 6db attenuator between it and the power divider. The data that I'm receiving is all pretty distant and week so I don't have any attenuation other than the power divider on those receivers. This setup has worked quite well so far. If I hook a regular scanner directly to the antenna feed I get LOADS in intermod. I also realize that having the preamp closer to the antenna is a better option but with only a 30Ft. run of LMR-400 I didn't think the benefits outweigh the challenge of having a preamp out in the weather.
Ubbe
Member
If it's a power divider then it has no isolation between ports? I think you have a CATV splitter with some 10-20dB isolation between ports? I have pretty much the same setup, for different antennas and some are combined, using 15dB amplifiers and 1to6 splitters or less with attenuators and 30ft RG6. I also have the amplifers in the room but tested directly at antenna and frequencies in the tuned band didn't change but out of band was improved.
My setup change so often that I haven't settled for one that works best. It's often one setup and then the next week another to be able to monitored different frequencies the best. Everything here are coming in at -80dBm as best and usually its down at -100dBm. The signal strenghts are very weather and season dependent.
/Ubbe
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