1/12 wave coaxial transformer: a 75 ohm to 50 ohm converter - 3rd May
I have some very good quality 75 ohm coax cable that I will be using on a 4m dual dipole antenna (more details below soon).
Here I will be phasing up the two crossed dipole to get various polarisations using switchable coax phasing lines.
Now feeding a 50 ohm system with 75 ohm should only create a 1.5:1 VSWR, if there is no reactance present.
But some reactance (and additional resistance) may creep in via the matching / phasing circuits - in which case this circuit will help.
PIC control board for SV1AFN band pass filter unit - 6th May
I am using a PIC16F877 to control a SV1AFN BPF unit (more details soon). It's rather 'over the top' to use this PIC to control such a simple circuit
but I happen to have a batch of them and it also means I can use a very powerful PIC BASIC complier, which makes development really easy.
The other board on the right is a 5V PSU board which also has space for the RS232 unit required for in-circuit bootloader programming.
Experimental 4m band crossed-dipole & phasing unit - 1st - 13th May
I built a set of crossed dipoles (45 degree and 135 degree) for the 10m band but 'plugged' in elements for the 4m band.
These are wired using high quality pair of 75 ohm cables into a phasing switch unit.
By switching in various lengths of 1/4, 1/2 and 3/4 wave sections I can get: 45 degree slope, 135 degree slope, vertical, horizontal and left and right circular polarisations.
Everything is done at 75 ohm (using 50 ohm quarter wave sections to transform the two 'parellel' dipoles back to 75 ohm.)
As its hard to get perfect phasing harness lengths and perfect switching, I kept everything at 75 ohm and then used the 1/12 wave transformer (above) to make 50 ohm.
The bench-test shown here uses two 75 ohm dummy loads (purple) but early tests show the system also works with the dipoles.
(Phasing circuit from VHF-UHF handbook, 2nd Ed. 2013 Ed. Andy Barter, ISBN 9 781905086313 page 158)
3D printed center holder for light weight 4m band crossed-dipole - 16th - 21st May
The crossed-dipole set-up for 4m band shonm in the 1st-13 May entry was a modified 10m band anttena and was simply too heavy to go on top of my fibreglass mast.
So I 3D printed a X-piece to take small aluminium rods that cam hold 1m long Al welding rods for the dipole elements.
The result is something much, much lighter than the original. VSWR on the set-up very good, less than 1.5:1 on all polarisations.
Note: the 'old' 10m band baluns (coiled coax) are still in place.
.... More ideas coming soon include: DRV8825 bi-polar stepper motor driver, capacitance hat for helical LF antenna (?),
op-amp Analogue Computers, Energy Harvesting unit, switched capacitor laboratory pre-amplifier,
HF 5/8 wave vertical with coax-matching circuit, using a LMC6041 cmos op-amp to measure Circadian Rhythms, ...
click here for my 'X-files' page (other 'interesting' projects / ideas)
THE CREATIVE SCIENCE CENTRE
Dr Jonathan Hare, The University of Sussex
Brighton, East Sussex. BN1 9QJ.
home | diary | whats on | CSC summary | latest news