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AO40 The Easy Way by VK3CJS
CONIFER MODIFICATION - Page 3
| From the previous page, you can see that the lower frequency converter is redundant. Suspecting that it may be adding a little unnecessary noise, I severed the link between this converter and the IF preamp. Bingo! Just that little bit better. I cut through the circuit board track. You could just as easily crunch the last coupling cap which sits just to the rear of the mounting screw. |
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The UHF (IF) filter is made with normal air-wound coils. I spread the coils to bring the IF passband up in frequency and centre it more nearly on 450 mhz. Another s-point!
I adjusted the coils in the shack by rigging up a computer crystal module as a signal source. Use a 66.667 mhz oscillator and a diode for a multiplier. The diode is the "antenna" too.
Just sit it in front of the Conifer..... the 360th harmonic comes through clear on 2400.0
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Next a solid offset feed PAY TV dish was tried. Background noise was noticeably lower as predicted.
Using VK1KRF's suggestions a helical feed was tried. The webpage is HERE. A 5.25 turn helical was suggested as the best size for the job by G6LVB HERE.
The circular reflector in the picture is a coffee tin lid. The centre stem is a riser tube from a sprinkler system. The standoffs are small cable ties with holes drilled at the correct radius for the coils to pass through.
There is noticeably less QSB when listening with the helical feed compared with the original dipole.
If you can't steal a solid dish, try making one of THESE.
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