Friday, January 6, 2017

17M Moxon Update

Having built the 17m Meter Moxon and tested the antenna I am very pleased with the results.
Attaching to a 35 foot mast with a rotator however was touching its limits. The mast is in fact one standard 30 foot with a 5ft section added and a smaller section housing the rotator. Normally the mast is held by a large nut and bolt between two smaller lengths of scaffold pole that have been buried two feet in to the ground, therefore providing a clamp to the main pole that can then swivel up and down.

Once in place I can clamp with another nut and bolt that slips in at the bottom, locking the whole thing in place, it worked fine for my 10 meter Moxon but this is a bigger beast.

The problem is that combined weights of the rotator, antenna and cables puts the 35 foot mast at its limit. As I raise it bends quite dramatically and although it does settle when at full height I can't take the risk of keeping it in that position.

So I am going to have to reduce the height of the mast by 5ft i.e. remove the added section, this should strengthen the pole, its just slightly shorter, the attachment is added by a simple scaffold clamp, but the clamp does provide some weakness to the overall mast.

I can live with a 5 foot reduction so long as I know its safe! So at the moment the antenna is lying low with work to reduce the pole on-going. The cable for the rotator arrived the other day and I have already unwound and laid it out, connected to the rotator and it is now awaiting me to insert through one of the walls in to the shack.

The antenna and pole resting!
With the weekend upon us, I did some final testing I was getting an SWR of 1:2 at 17.185 and a 1:9 at what I wanted at 18.068 right across the band. So it needed a bit of a trim. What I found out later was because I was using pvc covered wire (in fact it was 42 strand speaker cable at around a millimetre width) I inputted the millimetre width within the Moxon calculation, I should have allowed for the PVC cover, so in future this has been noted as I wouldn't mind attempting another Moxon at some stage.
Further testing with the pole at rest
Anyway I trimmed using the 17.185 as a template, When completed I had shortened the director by 150mm and taken 50mm off the  reflector. This brought the SWR to 1:3 at 18.100, so all was well in the end, just a bit of give and take. The height is now 25 Foot and doesn't seem to be a great difference performance wise, only it feels and is a lot safer to put up and stay up!
You can definitely hear the difference when rotating, your up against the cobweb, which is not weak but there's certainly a 3/4 db improvement with the Moxon.
The completed antenna

It's taken roughly a week to construct, but it's been good fun and if your looking for a quick project I highly recommend having a go with a Moxon.

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