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wire dipole for 2mt/70cmt help needed
Posted: 08 Feb 2018, 17:09
by WARLOCK
i am thinking of putting a wire dipole for 2mt and 70cmt , i have seen people say 18 inches up and 18 inches down would this be about right ???
Re: wire dipole for 2mt/70cmt help needed
Posted: 08 Feb 2018, 17:31
by DX-Digger
1/2 wave dipole (feet) = 468 / frequency in MHz.
:.
468/145MHz = 3.227ft =38.7inches so aprox 19 5/8" inches each leg.
468/435MHz = 1.075ft 12 3/4 inches so 6 3/8" inches each leg of dipole.
You will have to trim it for exact match I wouldnt use thin wire though I would use say 10mm or 3/8" aluminium tube the thicker the tube the wider the bandwidth it covers and the shorter it will need to be trimmed for a good match.
Hope that helps
Re: wire dipole for 2mt/70cmt help needed
Posted: 08 Feb 2018, 17:55
by ch25
I recommend that solution:
Chris
Re: wire dipole for 2mt/70cmt help needed
Posted: 09 Feb 2018, 09:38
by WARLOCK
Thanks Chris ,i know this makes me look thick but can you please give me a little more details as to build this antenna , I do not understand the driven/sleeve elements ???
from George - warlock
Re: wire dipole for 2mt/70cmt help needed
Posted: 09 Feb 2018, 12:39
by ch25
I built this antenna from 1/4 inch copper brake pipe widely available.
2 plastic water pipe pieces to keep sleeve elements in proper distance.
You just drill 2 holes in plastic pipe and put copper elements through.
You connect coax to longer elements, hot to the top, braid to bottom. Do not forget to put some ferrite clamp-on cores on coax. We do not want it to be a part of antenna.
Chris
Re: wire dipole for 2mt/70cmt help needed
Posted: 09 Feb 2018, 19:55
by paulears
The ¼ wave for 2m is a surprisingly good match for a ¾ wave on UHF - very simple to produce, and very cheap.
As for ch25's design - it is a ¼ wave for 2m, with two parasitic half waves for UHF - I'm not quite sure what the radiation pattern would be, but certainly not circular - not sure what it's advantages are - it doesn't appear to have too many advantages, and mounting it would be interesting?
why do you recommend it?
Re: wire dipole for 2mt/70cmt help needed
Posted: 09 Feb 2018, 20:17
by ch25
Just quick modelling in Mmana.
145 MHz dipole will give 75 Ohm, j6 1.5 SWR. Low elevation.
Same dipole on 432MHz 78Ohm, j-52 2.5 SWR. 48 degrees elevation.
GP with 90 degrees radials is even worse.
It matters on RX too.
Antenna I suggested can be tuned both bands. Omni radiation.
How to build? It looks like that:
http://sp-hm.pl/thread-2666.html
Also look at radiation pattern and Z and j
Chris