Saturday, February 1, 2020

20m deltaloop experiment

As you know I'm thinking of a alternative 20m antenna to use in the PACC contest. A deltaloop antenna could be good choice. After some advise from PF5X Enno I decided to use his design. He provided me with some fantastic pictures.

Total wire length bare copperwire 1mm diameter would be 22,6m. So, isolated wire like I use would be 22,6x0,95=21,47m. The point to feed the antenna would be at 5,37m a quarter wave from the top.

The weather was great, dry with moderate wind. Setting up a mast of 10m became too heavy unfortunately. So the mast was 9m heigh. The design of Enno should have a 90 degree corner at the top. This looks easy but it isn't since it would take a large length of wire to guide the wire in the right direction you need a lot of room for that, 45 degree is a lot easier. I think in the end I had a 60 degree corner.

As always building such a antenna looks easier but is more complicated as you think. Wires that get wrapped up, cable ties that break.....but in the end the antenna was standing. Measuring with the MFJ analyzer revealed a SWR of 1:2 at around 14,2MHz. R=32 X=30. Not really what I thought of. This was the best SWR point. Enno told me I should look for a X=0 for resonance. But the nearest X=0 was at 15,6MHz. So, I don't know what to think about it? However, I tried to move the feed point a little but things didn't change at all. I also flattened the delta loop a bit to see if anything would change, but it didn't help much as well. Anyway it started to get dark already so I ended the experiment. My daughter did draw a nice impression of the antenna in the meantime... I connected the coax to the shack so I could get a impression of the antenna.


Deltaloop design pictures from PF5X.
Back in the shack I found 20m propagation just fading away, early this evening. I checked SWR and found a 1:1,5 this time, probabely loss from the long coax cable. I quickly compared the deltaloop with the vertical on top of my tower. I immidiatly noticed a lot of noise on the deltaloop, expected since this is coming from the neighbours. But unfortunately the noise does increase 2 S-units compared to my vertical, that is way too much. I compared signals between vertical and deltaloop, that was almost equal, most times the vertical wins.

So, this was a nice first experiment but not really a success. I will change the feedpoint to corner that is pointing to my neighbours and add a quarterwave 75 Ohm coax. This will give directivity but also will supress the noise hopefully. It will also add a higher radiation angle, something that I probabely need for advantage inside Europe. If this last experiment will not bring the improvement I need I will finish this project and set up the endfed again as second antenna in the contest.

2 comments:

  1. I cannot see the wires in the picture very clearly, but it seems that the feedpoint is too far away from the right corner of the the delta ... (about 5/6 from the top, 1/6 from the corner). Furthermore, if the loop really resonates at 15.3 MHz it is simply too short ... I guess X=30 should read X=-30, meaning the MFJ is only giving the absolute value of X and not its phase.
    Getting the top angle at 90 deg is indeed a bit of a challenge. What might help is putting poles as support at the left and right corner. That way the loop is easier brought into a right-angle triangle shape. Also, simply tie the middle of the base to mast at the correct height. This way you help lifting the entire base to the correct height and is the loop easier brought in shape.

    Keep trying ... Success !

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    1. Tnx for the comment Enno. I tried to lower the feedpoint further away from the top about 40cm. It didn't change anything at all. Strange because it was my theory as well. Then I tried to flatten the delta a little, it gave my a tiny bit better SWR but not that much. At 15,6MHz the R=7 and X=0. Very strange, I'm shure the whole antenna was not too short. Unfortunately I didn't have room enough to ad 2 extra poles. Anyway, more about my experiments in next post. 73, Bas

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Thanks for your comment. Bedankt voor je reactie. 73, Bas