Hello Barry and John,
Rodger on missing the first page of the last post I now repeat it:
1. Yes John your question about the gain being 7db over isotropic is valid, my measurements were not done "really" scientifically merely comparing signals over the long path from Alice to the UK where the signals were consistently S8 to 5db over S9 with 100W.
2. Back to the Astronomy antennas:
Here is a nice site :
http://www.radiosky.com/ here they said that the storms on Jupiter may be easily observed using a dipole antenna - Looks as if your path is right Barry and whats more I believe that you will be successful in this project - I might even have a go myself if I can lay my hands on a receiver.
3. Here is a dipole antenna that is very easily constructed. The dipole was calculated for 21,9MHz. This dipole is about 5% shorter because of the matching system I have chosen. It is mounted close to the ground so that it will direct most of its energy upwards.
It is stretched between two timber posts the nice part about this antenna is that it may be very easily modified for circular polarisation (installment #2).
Dipole antenna (receive only) for 21.9MHz.
|<------------------------------6.5M----------------------------->|
|<------------3.25M--------->|
|<-----1.5M--->| |<-----1.5M-->|
==============0==================== =====0============
| |
__0_____________.X._____________0__
^ | |
| | | <-------- 300 ohm lead
| |-|-|-| (on balun)
Sliding | | | <-- 300 ohm to 75 ohm TV balun.
contact | |--|--| (from DSE/Tandy/TV shop)
(welding rod) |
|
|
| 50 ohm / 75 ohm mismatch
|
|----------> To Rx.
1. Construct the antenna in bare copper wire
a single length of 6.5 M. and two 2M lengths.
(electrical earthing wire is good).
2. The matching section consists of 2 wires parallel with
the dipole separated from the elements by 100mm and a TV balun.
Use drilled electrical conduit(120mm x 12.5mm dia) as
the insulating spacers - 6 per side (say).
Use 2 pieces of brass welding rod each 120mm long with eyes
formed on the ends - these are the sliding contacts.
3. Start construction by getting a piece of 50mm dia. electrical conduit
200mm long and drill a hole throuh the conduit 20mm from the top for
the antenna wire to pass through. Drill 2 pairs of holes
(diametrically opposite) and 100mm below each of the previous holes.
The holes in each pair are 15 mm apart and are used to terminate
the matching wires.
Pass the antenna wire through the top holes in the conduit
and fix the wire so that the conduit is at the half way point
and cannot slide up or down the wire.
Fix the matching wires by passing them through the 15mm spaced holes
then twisting them - provides a strong mechanical joint and a good
insulator in the centre to which you attach the TV balun
and the co-axial cable.
You now have a piece of conduit to which are fixed 3 pieces of wire;
the continuous antenna wire and the two matching wires being
electrically separated.
Next thread the spacers through the wires then the brass rod.
The antenna should look like a long piece of wire with the conduit
in the centre and two ladders, one on each side.
Install the balun. The 300 ohm leads connect to the matching section as
shown and the balun is fixed to the centre conduit.
Connect the co-axial cable to the balun and fix the cable to the centre
conduit with a cable clamp. The antenna is now ready for installation.
4. Mount the antenna so that it runs N-S and 3.3M above the ground
using insulated (wood / fibre glass) posts.
5. To adjust; connect the antenna to the radio tuned to 21.9 MHz (approx.)
Adjust the tap spacing (shown for example at 1.5M) symmetrically until
the received noise is at a maximum. These rods are slid back and forth
symmetrically while monitoring the receiver noise when the noise peaks
you have a good match.
When the best match is found cut off excess wire from the matching wires
re-test the antenna then tidy up the job and that's it!
Kind Regards,
Jerry.
Thanks Jerry. You have given me much to think about. When a little spare cash is forthcoming, i may be able to walk that path.
