J pole with feedline impedance other than 50 ohms

[wa2ise]

Oh, there's tons of instructions on how to make a J pole antenna fed by 50 ohm coax. But there's absolutely nothing I can find if you want to use feedline of some other impedance, say 75 ohm coax, 300 ohm twinlead, 450 ohm ladderline, or even a twisted pair in ethernet cat 5 cable (100 ohms balanced, I ran some to extend a POTS phone line, and had a leftover twisted pair, and hey, Cat 5 runs in the VHF spectrum anyway, but not on this cat 5 cable or else the FM radio would suffer too much leakage). This must be classified or something...

As a scientific wild @$$ guess (SWAG) it looks like you can locate the feedpoints for these other feedline impedances by taking the square root of the ratio of (desired impedance/50 ohm), and taking that resulting number and multiply it by the length from the bottom (shorted end of the J pole) the instructions tell you to place the 50 ohm coax connections. It seems to work on a J pole I made for my FM broadcast receiver, though I realize there's a ton of variables, like receiver mismatches, stray conductors near the antenna and so on.

Yes, I know that all ham rigs are 50 ohm, and thus why noone ever says anything about any other impedance location. But if I wanted to make a J pole to receive say an FM broadcast station at 101.1 MHz and couple it to the 75 or 300 ohm FM tuner input, I'd want to know where these impedances exist on a J pole antenna.

[JBL GUY]

Edit, either it is my meds now or when I wrote the post, but it seems that I can't even run my calculator maybe.

I will recheck it and repost.

[wa2ise]

Hi JBL_GUY, this diagram I think would summarize it:


Though this ratio must break down at some point before we reach the top of the stub, where the impedance should be very high, much higher than say 600 ohms for a twinlead stub...

[JBL GUY]

Originally Posted by wa2ise

Hi JBL_GUY, this diagram I think would summarize it:

If you read my post that would have been in your email notification if you are subscribed to this thread, you will see that I explained the same thing in detail.

It is just that my medications and health got ahead of me and I was not even sure what I had posted, so I deleted the post. I wanted to be sure that it was not full of errors.

I do have a copy of it as a word document.

I understand how to do it and I made an Excel spreadsheet that does a very good job.

I have modeled this and many other antennae in software and just between you and me and I will not name the company, but back in the day I worked for one of the larger amateur radio and commercial antenna manufacturers.

I have even been to the Dayton Hamvention on the companies dime, first class all the way. Ah, the good old days when I was more than fifty percent alive.

Back to my spreadsheet, it takes into account some things such as but not limited to...

The antenna impedance ZA normalized to the impedance Z0 of the transmission line used for the matching network.

The normalized (inductive) reactance of the shorted stub section.

Antenna Feed point Impedance of the half wave radiator portion of the j-pole antenna.

It is not very user friendly.

My medications are getting ahead of me again and this is one of my sleepless periods, I have been up for 27 hours.

I may repost my post that I posted after I recheck my post that I posted prior to reposting the post that I posted.

It is time for me to stop trying to post for awhile.

Thanks for the reply.

[wa2ise]

Originally Posted by JBL GUY

...
My medications are getting ahead of me again and this is one of my sleepless periods, I have been up for 27 hours.

...

It is time for me to stop trying to post for awhile.

Thanks for the reply.

No worries, take care of your health. I'm going under the knife myself Dec 2nd...