Sunday, February 3, 2013

Both Cobwebs Now Up and Running

Well after much playing about with tuning and trimming, I finally got the G3TPW Cobweb back up on the mast yesterday evening.
After my initial tuning the other day I had to dismantle the antenna as the weather was proving very bad, so yesterday by the time I had re-assembled and started up the tuning process the morning had passed quickly by. Finally with a bit more trimming completed, I climbed the ladder with antenna in hand and clamped it in to place.
I switched on the MFJ 259 and did some checks, the SWR was all over the place, I had a problem, something was definitely not right and after a quick check of all the coax cables, I had to make the decision to bring the antenna back down, and so I clambered back up the ladder and removed the antenna.
I spent most of the afternoon with the terminal box open trying to figure out what the problem was, I knew it was shorting somewhere but in the end I had to go through each connection to find the short. Eventually I found the problem, it turned out to be one of the 12 meter elements had split at the join to the choc block causing one of the U connections to short and all it needed was re-stripping. Then it was just a case of re waterproofing the box, sealing up and mounting back on the mast.
Dusk was fast approaching when I clambered back up the ladder and attached the cobweb back on the mast, I quickly said a prayer and did my checks on the MFJ and thankfully all proved OK. 

Finally back on the mast!

The SWR measurements are now pretty good, they are all set for CW except for 10 meters which I set for 28.500 as I like an occasional rag chew on 10.
14.000 - 1:4
18.070 - 1:0
21.050 - 1:0
24890 - 1:2
28.500 - 1:1
With the G3TXQ cobweb now set up in the attic I am at last ready to do a bit of testing with both cobwebs. Obviously as this antenna is in an enclosed space there is bound to be a difference. The most noticeable being the background noise but that cannot be helped as the attic naturally has bricks and mortar and electrical wiring.
Here is the final SWR reading for the G3TXQ:
14.075 1:5
18.080 1:4
21.073 1:3
24.890 1:1                        
28.015 1:1    
As suspected there is not a lot of difference, but the SWR is slightly higher on the G3TXQ on a couple of bands (most likely because of the attic positioning). I could trim to match the frequency of the G3TPW, but to be honest I couldn't be bothered to get it exactly the same. 

Physically the G3TXQ is far lighter and easier to handle also the tuning is much quicker because you only have one wire for each element. But that said, I do like my G3TPW, it is heavier and chunkier with my larger home made plates and it definitely feels more solid especially with the windy condition we have round here.

It was very apparent when lifting back on to the mast how much heavier it was compared to the G3TXQ, you feel the lighter cobweb would literally blow away in the wind, although that said it's exactly the same build just lighter materials.
The main difference on receiving is of course the background noise but if the G3TXQ was raised outside on my mast I doubt you would know the difference when listening. On transmit I've now used both cobwebs on 20 meters and reports from both are good with the G3TPW showing  slightly stronger RSTs, again no doubt due to due location.

So all in all as suspected there is very little difference, for me I'd go for the G3TPW simply because it feels stronger, but that personal choice from QTH circumstances. It's been fun servicing the old cobweb and building a new one and I hope you have enjoyed the entertainment as I've struggled along. I'm not a technical antenna geek so all this is very basic but hopefully gives you a rough guide to both types of the antenna.

I'll probably remove most of the posts I have regarding the Cobweb antenna and place them on a separate page so that potential builders have all the information together.

I'm not sure what I'm going to do with the G3TXQ cobweb now, so for the moment it's sitting snugly up in the attic out the way. Currently I'm saving up for either a K2 or a TS590 so I'm tempted to keep the G3TXQ and connect to the new radio, then I'll be spoilt for choice with two cobwebs!

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