The cable to the Cebik Moxon aerial in the attic is RG58 and has a loss of over 3dB so I’m considering putting in a masthead preamp.
The cable loss was measured by sending the AW07A antenna analyser signal at 145.8MHz down it and measuring the RMS voltage at the HP 54615B scope. This was 194mV. The AW07A signal direct to the scope was 292mV. So the voltage loss was .
However, it’s not just a matter of inserting the preamp into the signal path. It needs powered somehow and protected from transmit RF.
Here’s some scribbles from my lab book showing my initial thoughts on how to do this. I got an M–100 preamp for Xmas so I’ll use that.
I thought it was about time I understood aerials more, so I am building an omnidirectional aerial for LEO satellite use.
I’m hopeful that I will be able to use the aerial in my attic, but if not I’ll use it in the garden. The aerial I’ve picked is documented in the ARRL Antenna Book in one of the supplementary PDFs. The article which describes it was published in QST in 2001 and is called “A Simple Fixed Antenna for VHF/UHF Satellite Work” by L. B. Cebik, W4RNL. I can’t link to it because it is copyrighted by ARRL but I explain most of it below. It is a pair of Moxon rectangles at 90° to each other.
I made a model of the aerial in the excellent Mac application cocoaNEC by Kok Chen, W7AY. The other aerial modelling applications I looked at were all based on spreadsheet tables. I found the programming approach used by cocoaNEC easier to use. Here’s the shape of the aerial as shown by cocoaNEC.
The driven elements are the top ones, and the reflectors are the bottom ones. Both have vertical parts which make up the Moxon rectangles. One of the driven elements is driven from the feeder through a matching line. The other driven element is driven 90° out of phase through a phasing line. This gives the radiation pattern below. This is the VHF version of the aerial. As you can see it is almost omnidirectional but favours elevations that you might expect to be able to use a satellite at.
I was given ‘Learning the Art of Electronics’ by Thomas C. Hayes for my birthday. This is a hands-on lab course associated with the Horowitz book ‘The Art of Electronics’ (AoE). I am working my way through this and am hopefully filling some of the holes in my knowledge. It’s certainly made me think more deeply about some of the electronics I thought I understood! It doesn’t just rehash the AoE book — it approaches the theory in its own way. It’s good fun, so far.