23cm SEPTUM FEED       (Note:   Below describes my very first septum feed.   For the latest new teqhnique:  http://kl6m.com/23cm/23CM-SEPTUM-FEED.pdf      )                                                                                                                                        More photos, with scalar ring:  http://kl6m.com/23cm/scalar.html

Return loss charts:       cursor = 1296.0 MHz      marker = 1147 MHz  ???

I'm not sure why there seems to be a natural resonant point at 1147 MHz.  This did not seem to change when monopoles were lengthened.

Isolation before lengthening was about -16 dB.  The plot showed a much deeper isolation (>>20dB) about 40 MHz wide at about 1375 MHz.  After lengthening the monopoles isolation worsened to -13 dB.  I tuned for maximum isolation and the best I could achieve was -22 dB.  At this setting the return losses were very bad, around -8 to -10 dB.

I plan to try some different lengths and perhaps some different diameters for the monopoles.  I am also questioning whether or not I should terminate the other port during return loss optimization.  Normal transmit operation would have the RX port unterminated.  Normal receive operation would have the TX port terminated in the coaxial feedline all the way back to the amplifier.

Comments are welcome !!

 

1a-probe.jpg
1a-probe.jpg
61.16 Kb
Monopole probes
6mm diam. x 43mm
soldered about 3 mm
from connector
1b-probe.jpg
1b-probe.jpg
59.24 Kb
Monopole probes
after resoldering
about 5mm above
connector
1RL.jpg
1RL.jpg
66.67 Kb
1st measurement
Return Loss
Left port
2RL.jpg
2RL.jpg
65.05 Kb
1st measurement
Return Loss
Right port
3RL.jpg
3RL.jpg
43.36 Kb
Left port
Return Loss
retuned
4RL.jpg
4RL.jpg
62.97 Kb
Left port
Return Loss after
lengthening. No
termination on
right port
5RL.jpg
5RL.jpg
63.10 Kb
Left port
Return Loss after
lengthening.
Termination on
right port
6RL.jpg
6RL.jpg
62.59 Kb
Right port
Return Loss after
lengthening.
Termination on
left port
7isolation.jpg
7isolation.jpg
68.90 Kb
Isolation between
ports. Max isolation
is occuring at
1375 MHz
_septumparts.jpg
_septumparts.jpg
56.70 Kb
Fabrication of
parts completed
Ready for assembly
quick70cm.jpg
quick70cm.jpg
39.83 Kb
New feed support
with temporary 70cm
using HD73 for polarity
redoubt10-04.jpg
redoubt10-04.jpg
68.61 Kb
Mt Redoubt
As seen from our
living room window
5-Oct-04

 

23cm SEPTUM FEED - FINAL RESULTS

Final Return loss:  Right port: -27 dB (1.09:1 VSWR)  Left port:  -35 dB (1.04:1 VSWR)  Isolation: -29.8 dB

I had excellent comments and suggestions from moon net folks.  Most suggested that I start trimming and try to find the best results.  One person mentioned that their final monopole length was around 48mm.  Another mentioned 38mm.  So I started at 48 and started trimming.  When I got to 39.5mm I reached a maximum isolation of almost 30 dB!  So I backed up .5mm and soldered and bolted everything down and ended up with the EXCELLENT results above!

Mission accomplished.  THANKS!

1leftport-f.jpg
1leftport-f.jpg
68.49 Kb
2rightport-f.jpg
2rightport-f.jpg
69.27 Kb
3isolation-f.jpg
3isolation-f.jpg
70.75 Kb
rearpanel.jpg
rearpanel.jpg
74.17 Kb