Subj : Re: Homebrew antenna questions
To   : alt.ham-radio.vhf-uhf
From : Bob Bob
Date : Wed Sep 28 2005 09:05:43

From Newsgroup: alt.ham-radio.vhf-uhf

Hi Carl

http://www.cebik.com/vhf/jp1.html has a lot of info about Jpole design.

Generally speaking though the differring material will make a small 
enough dfifference to ignore. Paint has far larger an effect.

The smaller diameter means your elements will be slightly longer. There 
are length/dia graphs out there that can be consulted. My gut feel would 
be about 1.5% longer though.

The smaller diameter also means a slightly smaller VSWR operating 
bandwidth. I'd suspect this wouldnt be a huge problem.

The smaller diameter also means you have to scale the "open feedline" 
section spacing. (ie make it smaller)

Having just moved to a new house I put up a very quick and dirty Jpole 
made from galv steel fencing wire about 2mm dia. It is hanging from a 
tree with polypropelene rope. I tuned only by moving the feedline taps 
and it seems to work fine. Other may disagree but I feel the antenna 
dimensions (once presenting a suitable load to the TX) isnt that 
critical. A more troublesome effect might be feedline or mast radiation 
skewing the pattern upwards. I tend to use 4:1 coaxial baluns on J poles 
(it moves the feedpoint higher) but there are other mehods to decouple 
the mast/cable.

I will perform like a half wave vertical dipole! You can model 
performance if it is important to you. Other effects may be more 
significant though. (eg local terrain and antenna height)

The cable length will of course affect the range of the system. If you 
used something like the 1/4" RG58 cable the loss would be in the region 
of about 3dB. If you used the 1/2 inch cable RG8 or 213 the loss might 
be around 1dB. The above at 144Mhz. Depending on what you are trying to 
achieve these arent huge loss figures. Although over an LOS path you 
might see better range (you lose 6dB every time you double the 
distance), at a distance where your signal is already low and in clutter 
you wont see much of a range increase. (The reason being that the extra 
2dB will be lost very quickly in foliage etc) If you are serious about a 
really good setup then there are other places where you can make up for 
that loss (eg antenna heigh/gain, preamps and output power)

Regarding a single Jpole antenna for 6 and 2. I have only seen the 
stacked variety and have never tried building one. No doubt there are 
other designs out there. You may even be able to adapt the more common 
2m/70cm designs for the purpose.

Hope this helps.

Cheers Bob W5/VK2YQA


Carl J. Hixon wrote:
> 
> 
> Question for the group.  If I wanted to make a 2m or 6m j-pole out of 
> 1/4-in or 1/16-in 304 or 326 SS tubing, how would the change in material 
> and diameter effect the design formulas that I find on the web.  ....

.