G0FTD > ANTENA 06.03.05 01:51l 49 Lines 1405 Bytes #999 (0) @ WW
BID : 119708G0FTD
Read: DO3JB OE5AKM DK7JAN DG5YM DG8DG GUEST
Subj: A simple 2m yagi
Path: DB0FHN<DB0MRW<DB0WUE<DK0WUE<SP7MGD<IZ0AWG<IW8PGT<ON0AR<GB7CIP<GB7SXE
Sent: 050305/2254Z @:GB7SXE.#38.GBR.EU #:23708 [Hastings] FBB7.00i $:119708G0FT
From: G0FTD@GB7SXE.#38.GBR.EU
To : ANTENA@WW
Here's a simple 145Mhz yagi that I made from an old Band 2 FM broadcast
antenna.
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The director is:975mm
The dipole is:1000mm
The director is:1004mm
Spacing between dipole and director:300mm
Spacing between dipole and reflector:600mm
Use a coaxial stub of approx 135cm OPEN ENDED RG58 to match to 50 ohms.
The diameter of the elements is not critical (as some would believe!)
An average of 5mm is great.
There is little difference between the gain figures in a simple array
like this - however long yagi enthusiasts will vouch for it's importance.
Be careful when snipping the stub for best SWR - it will vary a little bit
depending upon manufacturer of coax. Start at 15cm and snip 5mm at a
time and check SWR. It may be best to have several lengths of coax to snip
so you can see how the best match occurs.
It usually makes little difference as you snip each 5mm and then all of a
sudden you see a dip!
The modelled gain (using MMANA) is 7.42dbd but I'd expect to see more like
6db due to slight imbalance of the driven element when feeding with
with an unbalanced coaxial feeder.
This antenna has been built and constructed by myself and performs as
documented.
- Andy -
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