Apr 262013
 

Firstly I would like to publicly thank the two Micks, Robert, Karen and Tony who have been down to the shack and had a tidying up frenzy after the work we have been doing of late. The cables on the tower have now got plenty of cable ties and have been routed to the appropriate radios in the trunking. They have also got rid of quick a lot of ‘junk’ – in fact the shack looks enormous as a result and has a distinct echo :)

I have forgotten one or two things that have also been done – Robert will put me right. One thing is for sure we are making rapid progress!

Thanks for your hard work everyone.

I took a rather posh Rohde & Schwarz  ZVL Vector Network Analyser up to the shack tonight to have a look at the new 50/70 MHz dual bander. The tuning is not quite right, more on that later. Anyway here are the plot, still perfectly usable but we can make it better.

Click for full size images:

Screen grab of the return loss (RL) on 49-53 MHz. Maximum RL (min VSWR) is 50.398MHz - little high

Screen grab of the return loss (RL or S11) on 49 – 53 MHz. Maximum RL (min VSWR) is 50.398 MHz – little high

RL (S11) for 70 MHz. Minimum is out of band and needs to be improved.

RL (S11) for 69-71 MHz. VSWR Minimum is out of band and needs to be improved.

You may be wondering what this Return Loss or S11 is. I will do a little article explaining it, how it relates to VSWR and even links to a tool so that you can convert one to the other. To put things into perspective with the return loss shown on 50.398 MHz if you were to put 100W into the antenna a little less than 5mw would be reflected. This is a VSWR minimum of 1.01:1, pretty good! We just need to move it a little lower to the DX portion of the band, although the RL at 50.000 MHz dead is still 23dB which means 100W forward 0.5W reflected – actually quite good enough.

On 4m however 11 dB RL at 70 MHz means 100W forward just under 8W back, that needs some improvement I think.

I am fairly sure that the HF antenna is too close to the dual bander so we are hoping to get a longer stub mast.

Justin G0KSC has also suggested how I can improve the way that the choke balun is mounted – only a 5 minute job fortunately.

Mick and I were talking about the luffing winch and head unit bearing loading tonight, if we were to extend the stub mast appreciably and use a steel scaffold pole. He is a bit of dark horse that one, he knows a lot of stuff about a lot of things. It seems that our Mick has done some fairly serious mechanical engineering at some point.  So we’ll work stuff out properly and do the best we can. We all have a feel for these things but its always better to do the calculations and find a use for those redundant fag packets. Or we could do a spread sheet but then that would be like being at work for me!

The HF antenna is working marvellously though!

  2 Responses to “VHF antenna – pre tweaks – quick report”

  1. Again a good report thanks Conrad,

    I am going to tease the Isaac Newtons of ARRL Tower Talk to see if they have any input on loadings.

    I am sure it is a question that will present itself again and this can be good source information for others following a similar path.

    Shack clear out going well but my garage is now full. I am off to Dambusters Rally at Woodhall Spa next saturday to try and get rid of a lot of it.

    Generator has gone and after listing extortion we have approximately £800 into club funds. Hopefully the rally will get us a bit more and be closer to achieving our goals.

    Ill keep you updated

    Robert

  2. thanks conrad for your report and comments for what you said about the work thats been done but its all down to your guys robert,mick p,tony,and yourself and mick d.i myself only hepled to clean the shack and got rid of all unwanted items the shack looks a lot bigger and better and to how it should look .so once again thanks guys

    Thanks
    Karen M6KSD

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