CARC Buildathon Loop Evening
CARC Members enjoyed a busy evening on Wednesday 23rd July when at least ten members plus a number of helpers took up a challenge to build and test a small electromagnetic loop for 7Mhz during the evening. The Club seemed to be full of people wielding Stanley knives and soldering irons etc. doing all sorts of mischief to bits of wood and coax. At the end of it there must have been around ten or more members with working loops plus quite a few interested spectators as can be seen in the photographs below!!
The whole event was triggered by a challenge from Peter Head G4FYY contained in an e- mail circulated to the membership. Peter G4FYY invited members to build a loop to the instructions he supplied. The idea was for each of us to bring along the listed materials needed to put the antenna together on its simple wooden mounting base. These were all basic items with little expense involved. Peter had already made up six kits of parts for those who has been unable to get their own together in time.
Several people brought SWR meters or hand-held network analysers to aid setting up. Several compact HF rigs were also in evidence to allow on-air testing outside the Clubroom.
The basic kit comprises of a couple of lengths of surplus 75-ohm coaxial cable, one for the main resonant loop and the other for the smaller coupling loop and a few spring C-clips from the “Pound” shop to hold the whole thing together!
One of the clever things of this design is the use of just a single main loop made of 75-ohm coax with the iner of one end connected to the outer braid of the other. The trick is to arrange for the inductance of the single turn loop to resonate with the capacitance of the coax that forms the loop at the desired frequency. A section of the outer PVC sheath is then removed at the join to reveal a length of copper braid which can be compressed or stretched over the polythene inner to tune the loop. Simply sliding the loose braid backwards and forwards along the polythene inner allows the loop to be tuned from 7.0 to 7.1Mhz. The two loops are simply held in place on the wooden support with several mini-plastic C-clips. This arrangement obviates need for the usual wide spaced and potentially expensive split-stator capacitor.
All of the loops were set up quite quickly and several close range test QSOs in and around the club took place. One interesting effect was noted during setting up the loops was that there appeared to some effects from near-by people moving when it was then realised that due to the relatively close proximity of other loops only a couple of metres or so away, the loops were interacting!! Your scribe was trying to move the resonance which appeared to be fixed when we realised there was another resonance which was in fact moving as we moved the tuning braid and which was the true resonance of our loop!!
Post meeting tests showed that the loop would handle at least 100W without “fizzing”! I even tried 300W PEP but I would not recommend it - at least only from a very safe distance!!
Thanks to Peter G4FYY for organising the event and all doing all the work in getting the raw materials together and providing a most interesting evening. So folks, get to it and join the loop group and try it for you self!!
To view or download a PDF of the whole project illustrated from beginning to end with parts list, building instructions and guidance for the setting up and testing of the final antenna, click on this link.
CARC Annual General Meeting
Wednesday 27th - WBR Regen Receiver
A presentation given by Peter G4FYY on the construction of his latest project.
All are welcome to attend.
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