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G0FTD ANTENA @ WW 2003-06-15 06:29 3742 3 band indoor VHF antenna


Best viewed in a DOS editor to ensure ascii drawing looks correct.

                   G0FTD's experimental INDiX antenna (c)

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 and breakfast or hung from a tree ! The antenna is really a bi-square
modified with two "rungs" and lengths modelled using NEC software for
optimum all round perfomance. 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. I have an interest in self contained
indoor amateur radio stations, hence I developed this antenna as something
real and practical to get on the air. The antenna is a 1.098 x 1.846m
wire rectangle with 2 elements connected 10cm inwards as shown, and a 
centre rung where the feedpoint goes.

쳐컴컴컴컴컴컴컴10cm inwards ladder rung element
쳐컴컴컴O컴컴컴캑1.846m           O=feedpoint
쳐컴컴컴컴컴컴컴10cm inwards ladder rung element

Gain:                     	Impedance:           Polarization:

50  Mhz = 2.35 dbd               18-j887              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 and
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
-------------------->To feedpoint O

144Mhz matching network

---\/\/\/\/\/------->To feedpoint O
             ---                    L=0.107uH approx
             ---                    C=11.4pf approx made variable
-------------------->To feedpoint O

Alternative Matching networks using only inductors.

50 Mhz
Replace C with 0.118uH inductance
Replace L with a 2.667uH inductance

Replace C with 0.137uH inductance
Replace L with 1.386uH inductance

Not possible using inductors but a coaxial matching stub may be used.
Simply take a 13cm length of RG58 coax and attach to the feepoint 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 feepoint and
switch in line for each band.

Note: You must adjust the matching networks in situ and the impedance of
the antenna will vary slightly with proximity to nearby objects. 


You should get about 170Khz on 50/70 Mhz before the SWR reaches 2.1
but on 144Mhz the antire band should easily be covered.

On 50Mhz/70Mhz this bandwidth is quite adequate for working the DX ends
of the band.

So there you have it, another antenna designed to get amateurs on the
air no matter what circumstances are - and effectivly too !

Note 1:
70MHz is a UK/CYPRUS/GIBRALTER and (Croatian ?) band.

73 - Andy, G0FTD @ GB7SXE

Message timed: 00:24 on 2003-Jun-15
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