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G8MNY > TECH 07.01.11 08:33l 97 Lines 3599 Bytes #999 (0) @ WW
BID : 4229_GB7CIP
Read: GUEST
Subj: 3 Band VHF Indoor Aerial
Path: IZ3LSV<F6CDD<GB7CIP
Sent: 110107/0040Z @:GB7CIP.#32.GBR.EU #:4229 [Caterham] $:4229_GB7CIP
From: G8MNY@GB7CIP.#32.GBR.EU
To : TECH@WW
G0FTD's experimental INDiX antenna (c) (corrected May 06)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)
Indoor DX antenna. Meant to be hung indoors within a window frame for those of
you who can't get an outdoor antenna or operating from a hotel, bed & breakfast
or hung from a tree! The antenna is really a bi-square modified with 2 "rungs"
& lengths modelled using NEC software for optimum all round performance. You
have to make up a matching network for it which is easy enough. The antenna
can be made of wire with dowel rods to support the horizontal sections.
1.098m
ÚÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ¿
ÃÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´10cm inwards ladder rung element
³ ³
³ ³
³ ³
ÃÄÄÄÄÄÄÄOÄÄÄÄÄÄÄ´1.846m O=feedpoint
³ ³
³ ³
³ ³
ÃÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´10cm inwards ladder rung element
ÀÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÙ
Gain: Impedance: Polarisation:
50 MHz = 2.35 dBd 18-j870 Horizontal
70 MHz = 1.14 dBd 30-j633 Horizontal
145 MHz = 4.36 dBd 185+j102 Horizontal
Radiation pattern is just like a half wave dipole figure of eight pattern but
with slight gain. There is very little high angle radiation.
The antenna is a loop 1.846 x 1.098 metres with 2 "rungs" connected 10cm
inwards which improves the gain on 145MHz by approx 1 dB. An L-Match is
inserted at the feedpoint `O`. At 50/70MHz a step down l-match is used & at
145MHz a step up. All matching networks are placed AT the feedpoint, they are
*never* placed at the rig end.
50/70 MHz matching network
----/\/\/\/\/\------>To feedpoint O
|
--- L=3uH tapped for 50/70MHz
--- C=20pF to 150pF approx variable
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-------------------->To feedpoint O
144MHz matching network
---\/\/\/\/\/------->To feedpoint O
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--- L=0.107uH approx
--- C=11.4pF approx made variable
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-------------------->To feedpoint O
Alternative Matching networks using only inductors.
50 MHz
Replace C with 0.118uH inductance, Replace L with a 2.667uH inductance
70MHz
Replace C with 0.137uH inductance, Replace L with 1.386uH inductance
144MHz
Not possible using inductors but a coaxial matching stub may be used. Simply
take a 13cm length of RG58 coax & attach to the feedpoint but leaving the far
end open. Trim the length which should be around 11.5cm long but may vary
slightly. The coax can be coiled up to fit it in a box.
All matching networks can be made in a compact box at the feedpoint & switch in
line for each band.
Note: You must adjust the matching networks in situ & the impedance of the
antenna will vary slightly with proximity to nearby objects.
Bandwidth.
You should get about 200kHz on 50/70 MHz before the SWR reaches 2.1 but on
144MHz the entire band should easily be covered.
On 50MHz/70MHz this bandwidth is quite adequate for working the DX ends of the
band.
Note:
This article may be reproduced on a not for profit basis, personal use or
amateur radio club website or personal website. I've taken to putting this on
the end of some of my work as I often get asked if it can be reproduced in the
above media. It would be nice if anyone does reproduce it to keep me informed
so I can point others towards it. Also if anyone uses the antenna it would be
nice to know how you get on with it.
- Andy - G0FTD@GB7SXE.#38.GBR.EU
Why Don't U send an interesting bul?
73 De John, G8MNY @ GB7CIP
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