- Posts
anyone point me to a construction page (or provide dimensions) for a 450 ohm ladder line 5/8 wave j-pole antenna for 97.7 mhz to work with a 75 ohm impedance?
anyone point me to a construction page (or provide dimensions) for a 450 ohm ladder line 5/8 wave j-pole antenna for 97.7 mhz to work with a 75 ohm impedance?
I have lots of RG6 75 ohm coax and 450 ohm ladder line. need 75 ohm not 50 ohm impedance.
Total posts : 45366hello, test, test, this thing on???
Total posts : 45366Why noy just build the j-pole and not worry about the 75 ohm coax issue. The worst case is a SWR of 1.5:1.
Total posts : 45366the power is adjustable from near zero to 2mW into 50 ohms 10mW to 75 ohms. at those miniscule power levels it makes sense to get every bit of power by uses a matched antenna.
it’s actually outputting 45dbmv into 75 ohms. i looked up a conversion which stated that equals 10mW into 75 ohms 2mW to 50 iirc.
Total posts : 45366i am very surprised. there are 3 people here who i KNOW have the technical background to answer this question and all 3 have posted here since i posted this and the question seems to have gone unanswered.
Total posts : 45366“the power is adjustable from near zero to 2mW into 50 ohms 10mW to 75 ohms. at those miniscule power levels it makes sense to get every bit of power by uses a matched antenna.”
You will never hear the differece on the receiving end.
Total posts : 45366well i tried a search and could not locate a 5/8 wave 450 ohm ladder line jpole design for 3 meters anywhere. found plenty for 2 meters.
Total posts : 45366It is probably the illegality of the device you want to make that makes exact instructions scarce on the Internet.
Since the NOUO will be issued to you, and not to me, and you would be the one who might lose his ham license, what I would suggest (for information purposes only) is that you construct the 2-meter version of the antenna you want and get it to work. Then substitute 75-ohm line for the 50-ohm line. The coax type should not make much of a difference. Getting the antenna to work may not be an easy task because it is not that simple to get J-poles to work, even with instructions, and you may have to do some tinkering.
Then build the FM version by increasing each vertical dimension by the ratio of wavelengths, which will be somewhere around 1.48. You probably will not have enough power to get your SWR bridge to work on FM, so testing and adjustment will not be as easy as on 2 meters.
Total posts : 45366The 450 ohm characteristic of the ladder line has no bearing on its use for a J pole since except for the gamma match at the bottom only a single wire is used above that.
I built a couple of J poles from twin lead and the results were disappointing. It was very difficult to get a decent match and it was a critical adjustment requiring adjusting both the length and the feed tap point. Add to this the complication of a 5/8 wavelength which gives a high feed point impedance it may not be worth your effort. Even when I achieved a match it didn’t radiate as well as a simple 1/4 wave over radials. I threw it away.
No harm in trying and if you get it to work let us know how it went.
Neil
Total posts : 45366In the CC post by Ultanium, he states that he has a working J-Pole on the FM band. Perhaps he could shed some insight on the subject.
Total posts : 45366neither 2 nor 10 mw with an indoor antenna of any kind has a fat chance in hell of going anywhere. especially in a reinforced concrete and steel and bick structure to which im on the bottom floor and middle of building.
Total posts : 45366i give up on this idea. just spent an hour cutting the dipole making the matching stub and that effing aluminum shield on the RG6 wont solder it just beads right off of it and melts the jacket!!!!
Total posts : 45366deleted
Total posts : 45366Robert, I bought my heavy-duty broadcast version of the 5/8 wave j-pole years ago from the guy that still sells the 2M version on eBay, KB9VBR. To this day, he still shows the picture with the feed line going the wrong way, and for anyone that asks, the center conductor goes to the longer element, it is the radiator. That being said, there are others on eBay selling broadcast-band ready j-poles, or build your own from http://www.aralb.org/Commitees/Technical/J-POLE/design_a_jpole_antenna.htm
I have never tried ladder line, don’t see why it wouldn’t work with stiff copper wire, EMT, or even speaker wire stapled to the wall. All I can say is the darn thing works great with the little EDM transmitter, and from inside my garage with the door closed, is getting out between 3/4 and 1 mile in every direction.
Tommy J.
AM-580 The Bull!
Total posts : 45366thanks tommy.
i’ll tell you tommy i’m getting highly annoyed and tired over many things these days one of them being armchair lawyers and kilocycle cops up on their high horse preaching from behind a keyboard and it being so darn hard hard to sell anything these days to get money for upgrades without having to go to ebay and getting bent over a stick by them with theirs and paypals fees!!!
very frustrating to say the least.
i want to go back to 10-12 years ago when whatever info you asked for you got, and it was nice and quick and smooth to sell online!!!!.
these frustrations have come close to causing me to just say eff it and give up the hobby entirely.
i’ve always liked you and a select few others who have no issue sharing what they know without the preaching and helping others. it’s people like you that i do keep going in the hobby.
off my soapbox now.
- You must be logged in to reply to this topic.
