J-Pole, coax, and bnc, good grief.

[benpsut]

What's up guys. Just for kicks I want to build a J-Pole for the railroad band, 160-161mhz., for scanning. Just to see how well it'll perform. I have no problem with the actual building of the antenna, except one part. The coax and such. What sort of coax would you recommend? I read all the tutorials online and all the pdf's and the helpful hints, but none seem to be persistant on what type of coax to use. Also, I need a bnc connector for my scanner hookup, I've never done connection installations on coax, is a bnc on a coax relatively easy to install? Also, I'm not sure if I want to solder the lead and shield onto the J-Pole, or use copper tube clamps to hold the wires in place so i can easily adjust if necessary. You know, just loosen, slide to where necessary for tuning, then tighten again instead of breaking the solder. Any help would be appreciated guys! If you need any more information from me to answer the questions just ask and I'll reply. Thanks again.

Ben

 

[rvictor]

The type of coax will depend in significant part on the length and nature of the run that you'll need to make. For longer runs you'll be more concerned about losses between the antenna and the radio. RG-58 is probably the most popular for short to medium runs and even RG-174 can be convenient if you're connecting to a hand held scanner where the flexibility is desirable and the run fairly short.

BNCs can be a bit of a challenge if you haven't done them before. There are a considerable number of varieties. The first issue is how the tip is connected to the center conductor of the coax. Some are soldered and others crimped. Soldering takes some skill while the crimped ones require an appropriate crimper but less skill.

The other aspect is how the back of the connector holds the coax in place. There are some that are referred to as clamp type which screw together. There are also those that are crimped. The crimped are easier, but again require a crimper. Generally you can get a crimper that will handle both the rear crimp and the pin crimp with the same crimper. There are also some that screw on to the end of the coax.

All of these varieties are available for both RG-58 and RG-174 except, I believe, the screw on are not available for RG-174. If you buy connectors, try to get a stripping and assembly diagram for the specific coax and connector that you choose.

I've attached a catalog brochure for one brand of connectors that may make things less confusing.

Hope this is helpful. I'm sure others may have additional views.

73,
Dick

 

[kc9neq]

railroad j-pole

i made a j-pole and a slim jim antenna cut for the railroad band. both antennas were cut to the middle of the band. they both work really well. i was very impressed with their performance. here is a link to some plans: N4UJW ANTENNA DESIGN LAB - Ham Radio Antennas - Design 0r Build Your own Ham Radio Antenna
i used copper tubing, and soldered the center conducter to the long side of the j-pole. i attached a 4 hole so239 chassis mount to the short side of the j-pole. i drilled a small hole in the short side of the j-pole and used one screw to attach the s0239 chassis mount. you can get the so239 chassis mount at radio shack. i originally fed the antenna with radio shack rg58u cable with pl259 ends. i cut one off and attached a bnc connecter to it. i have also used an adapter and they seem to work pretty good. i have since fed the antennas with 9213 and i can tell a difference. my antennas are mounted in the attic. i know they would work better if they were outside. i found that the j-pole and slim jim antenna both cut down on some intermod that i was having with my small radio shack antenna used with a bcd996t. i did not use a balun because it is a receive only antenna. they are easy and fun to build.

 

[sleebusjones]

KC9NEQ has it down. The SO-239 is a good, sturdy way to attach a feedpoint to a J-Pole. I used that method when I made a 6M (!) J-Pole out of 1" copper water pipe. That was a really great antenna until there was a windstorm and the darn radiator bent. My method of attaching to the antenna was to use two pieces of 1/4" copper tubing, flattened on the ends and soldered to the SO-239. The flattened ends were then soldered to the antenna and checked for SWR to find a good feedpoint. IMHO, I think soldering is going to be the best method of attachment. Clamps can corrode, then you'll loose gain on the antenna.

As for the type of coax, it really depends on how weak of a signal you are trying to work. If you have stuff banging in all the time, and it's located physically close to your scanner, then you really don't need anything special. RG-58 is fine. If you are working weak signals, and you are a long way from the antenna, you may want to go with type N connectors, and some Belden 9913. You can convert to BNC at the scanner. If you're somewhere in the middle, PL-239's and RG-8 will work. Just remember as you go towards higher frequencies, the 239 type connectors and RG-58 are going to be more lossy.

KC9NEQ, thanks for that antenna link there, that looks to be some good reading!

 

[benpsut]

Hey guys, got it built and everything. A few problems that is has. After erecting it on a PVC mast, I look up and see that the shorter mast is bent ever so slightly. While heating a joint close to it, I must have broke it's solder and it tipped a bit then resoldered itself when it cooled down. I didnt notice when I was building it. Another problem is tuning, I don't have a SWP meter. So I'm going to have to blind tune the thing with my scanner hooked up. I built it for the 160.800 mhz to be in between the other channels I listen too and I'm receiving 160.320 really good. So it's a bit off. ;-) Reception isn't all that great. So, it needs a bit of fine tuning but it works. Hey, it's a start. Not bad for my first antenna, it works that's all that matters. May not work as good as I hoped, but it works. I think what I'm going to do also is break the feed point solders and use copper clamps to move the feed point up and down freely to see if that helps. Radioshack didnt have RG58 (The antenna is right next to my room downstairs so the feed is like 15 feet) but the guy said that the RG6 they had was good enough....I don't know about that......but then again, it's working. I'll keep you guys updated on it to see what I find out and how I tune it. Thanks a bunch!

Ben

 

[W6KRU]

SWR is not going to make a great difference on receive if its not way off. I suspect that you won't notice any difference when adjusting the position of the feedpoint. RG6 is better than RG58 so don't worry about that. Here is a link that explains the difference in coax pretty well. The chart towards the bottom is especially helpful.

Strong Signals - Co-ax Cable

 

[sleebusjones]

With RG-6 you have an impedance mismatch. It's 75 ohm cable. RG-58 is 50 ohm, which is what the scanner is expecting.

 

[W6KRU]

With RG-6 you have an impedance mismatch. It's 75 ohm cable. RG-58 is 50 ohm, which is what the scanner is expecting.

That is correct and it would be a problem for a transmitter but the effect isn't an issue for a receiver.

 

[benpsut]

Well, in between posting this and the last post, I decided to mount the whole thing on top of the shed in the backyard, which is a higher elevation than the actual roof of the house. So I'm higher than the house right now. I didn't have the proper length though. We had another 50 foot roll of RG6 in the garage and I took a coupler, coupled the 2 together and i think I made it worse. Reception seems to be a bit better but now there's a crap load of fuzz when they come over the scanner. I know a balun is used for transmitting, but would it be helpful in scanning? Or should I forget the coulpler and go out and buy a roll of 100ft cable instead of connecting 2 50 foot rolls. OR.....should I strip both connectors and just solder the center leads and the braid?

Ben

 

[W6KRU]

Ben you don't need a balun and don't try to solder coax together. You can get 100' of RG6 quad shield and some connectors for pretty cheap at wally world. It sounds like you are picking up some noise from somewhere. Is the coax running adjacent to power lines anywhere along the way?

P.S. The wally world coax is better than the stuff at rat shack. IMO.

 

[benpsut]

theres no powerlines in the air, all underground. although, the coax is running right by the electric meter, so......hmmmm. Think that would be the issue? I know the reception problem is because of us living in sort of a valley. It's not a low lying valley though, but we are surrounded by some small hills. Also, I keep on picking up say....channel 8 on channels 6 and 7 very very faintly. So it might be built wrong, i.e. I cut it too short or long. The copper was cheap, I'll see if i can build another one. Thanks again guys.


Ben

 

[benpsut]

Alright, I found this interesting. I must have cut wrong or something. After drilling out a hole for the short and long end of the antenna and wrapping the lead and shield around each screw and screwing them in then soldering, I still didn't get any good reception. I mean, it's high up, feed point is good, connections are great, but still nothing more than i can get with my car mount or my rubber duckie. So for the hell of it, I listened to random bands. Low and high. I can pick up the range of 162-163 PERFECTLY CLEAR! Which is the weather band on for the area here. So....did I cut too long? Think I should trim some excess off?

Ben

 

[ratboy]

The dimensions on a recieve antenna aren't going to make all that much difference. You're probably just in a dead spot for the particular signals you're trying to pick up. Move the antenna around and see if it suddenly comes to life.

When I make scanner antennas, I always make them a little bit on the big side, as I never could tell any difference, and most of them I hung from the roof in the attic, so the extra length comes in handy if I have to whack the end of to redo it, as the hook I put on sometimes breaks after a long time.

 

[burner50]

For the feedline, Run RG-6.

SWR isnt gonna matter, and the loss is less than rg-58.

Not to mention it is readily available along with good connectors.

 

[benpsut]

Hey thanks guys! I have the antenna currently mounted on top of a shed pretty close to the scanner, but not close enough to get away with one piece of coax, so I have 2 pieces of RG6 connected together with f connector joint. I still don't think it's high enough. From the ground, it's a good 15-25 feet in the air. It's mounted on 8 feet of PVC pipe (just using pipe clamps to hold the jpole onto the pvc), which is in a bucket of concrete to hold it upright and sturdy on top of the shed, a good 10-12 feet in the air. Sounds high, but it's pretty close to the roof line of the house. What my fiances father suggested was to climb the tree in the backyard and clamp it on a branch. This tree is HUGE. I'll try that tomorrow to see if I can get any improvement. We had a nice clear night a few days ago and I could hear a good 20 some miles away, FAINTLY. I can hear 13 miles away through the hills and stuff clearly on any day. I'll try the tree thing tomorrow, and just TRIM some of the top off on both the short and long ends to see if that will help any. You think there is a better approach to an antenna just for scanning rather than using a jpole? Is strapping this to the pvc hindering anything? Sorry about all these "noob" questions, I'm pretty new to this, but find it really interesting, and fun to get into. Thanks for all the help again guys.


Ben