wire dipole for 2mt/70cmt help needed

General scanning discussion forum. Talk about anything to do with scanners, equipment, VHF/UHF reception and the art of catching those illusive signals!
Post Reply
User avatar
WARLOCK
Top Poster
Top Poster
Posts: 1227
Joined: 04 Jun 2008, 08:27
Call Sign: 26TM134
Location: cambridgeshire

wire dipole for 2mt/70cmt help needed

Post by WARLOCK » 08 Feb 2018, 17:09

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 ???

User avatar
DX-Digger
Radio Addict
Radio Addict
Posts: 751
Joined: 27 Jul 2009, 22:01
Call Sign: TM205
Location: ... - --- -.- . --- -. - .-. . -. -

Re: wire dipole for 2mt/70cmt help needed

Post by DX-Digger » 08 Feb 2018, 17:31

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
ʎɐqǝ uo pɹɐoqʎǝʞ ɐ ʎnq ı ǝɯıʇ ʇsɐן ǝɥʇ sı sıɥʇ

User avatar
ch25
Super Member
Super Member
Posts: 419
Joined: 03 Dec 2016, 11:07
Call Sign: n0call
Location: Poland

Re: wire dipole for 2mt/70cmt help needed

Post by ch25 » 08 Feb 2018, 17:55

I recommend that solution:
Image
Chris
You can't have too many antennas...

User avatar
WARLOCK
Top Poster
Top Poster
Posts: 1227
Joined: 04 Jun 2008, 08:27
Call Sign: 26TM134
Location: cambridgeshire

Re: wire dipole for 2mt/70cmt help needed

Post by WARLOCK » 09 Feb 2018, 09:38

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

User avatar
ch25
Super Member
Super Member
Posts: 419
Joined: 03 Dec 2016, 11:07
Call Sign: n0call
Location: Poland

Re: wire dipole for 2mt/70cmt help needed

Post by ch25 » 09 Feb 2018, 12:39

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
You can't have too many antennas...

paulears
Radio Addict
Radio Addict
Posts: 724
Joined: 10 Jun 2007, 22:41
Call Sign: G4RMT
Location: North East Suffolk
Contact:

Re: wire dipole for 2mt/70cmt help needed

Post by paulears » 09 Feb 2018, 19:55

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?

User avatar
ch25
Super Member
Super Member
Posts: 419
Joined: 03 Dec 2016, 11:07
Call Sign: n0call
Location: Poland

Re: wire dipole for 2mt/70cmt help needed

Post by ch25 » 09 Feb 2018, 20:17

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
You can't have too many antennas...

Post Reply

Who is online

Users browsing this forum: No registered users and 5 guests