Tuesday, September 14, 2010

Wire Hairpin Monopole?

Stumbled across it, you guessed it, on the internet (interestingly enough, Juan EA5XQ (*) also tried out the DCTL too). A mono-band simple to build aerial, you guessed it again, just to tempting for me, so I built one for 30m to test it on the grabber.

Reaching about in my stock of raw material, a roll of flimsy speaker twin-lead (non-HiFi, just the cheapest stuff). In order for me to roughly know what to do, the dimensions of the cable were roughly measure and hacked into my favorite antenna simulation program (MMANA). The software optimized the monopole's length to 7.6m.

This method of making a monoband aerial was performed in the middle of the night
  1. cut 7.6m of twin-lead speaker cable
  2. leads shorted at a first end of the cable
  3. leads split at a second end of the cable, opposite the first end of the cable
  4. done
The whole contraption is now connected to the "Ant A" port of the grabber's FRT-7700.

The hairpin monopole clearly provides much more signal than the DCTL (on the "Ant B" port) and much more noise too. So much more noise that signals are actually drowned in it... Have a look, 2028z I switched back to the DCTL. Check the faint traces at 1958z vs 2030z...

Hairpin vs DCTL

Compared to the DCTL, the hairpin monopole is a much easier build, even though the DCTL is not the toughest either.
I decided which will be my grabber antenna for the time being. Draw your own conclusions, either of the aerials has got pros and cons....

Next: test the TXing capabilities of the hairpin monopole.

(*) Interesting twist of history, I built my first DCTL in 1997 inspired by some list postings. Only since I re-employed the aerial for grabbing, I researched the internet again for it, in the hope of some more experiences. Juan's webpage made me aware of the monopole, so I tried it. Funny bit, I came to the opposite conclusion, at least for grabbing on 30m, the DCTL works much better. I also had some QSOs on 30m and 40m, using the DCTL. With the help of my 1997 DCTL the 40m mod of the warbler received the other side of the globe and my few signals were received from many stations.

UPDATE (18th September)
Had the grabber running for one night and one day running on the hairpin. The success was very limited. I tried matching w/ simple poly-varicons, well, I could tune it to 1.2:1 @ 50Ohm... on 20m... yes, twenty.... hmmm! From here on, the DCTL seems to be the mroe simple solution, some twin-lead and a balun.
There will be one more test, having the hairpin installed vertically.

2 comments:

  1. Could be that your Hairpin Monopole is in a stronger electric noise field than Juan's - just a thought.

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  2. Could be. Juan writes about regular QRP work, not low noise stuff. WSPR easily decoded signals using the Hairpin Monopole, the difference is only visible when severely digging in the noise.
    Additionally, the DCTL is placed completely outside the building, whereas the Hairpin started indoors, next to the noisy stuff, ending in a tree.
    As an idea, one could consider matching the Hairpin to 50Ohm unbalanced, and have it located outside... also just a thought ;-)

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