My top band dipole was hastily constructed for the 90% Eclipse in UK in 2015.
Report here by John G4YSS: G4YSS: G/TW-004 on 160m for the Solar Eclipse, 20-03-15
I bought a 100m reel of hookup wire, unrolled it and folded it in half, paced out 40m from the loose ends and tied it there for the dipole centre. That gave me the dipole top, and the remaining “loop” was turned into twisted pair for the feeder. My Elecraft T1 tuner forms a match, but it has never managed much better than 3:1.
It is far too big to work on at home, so today I took it out onto the Long Mynd G/WB-005 (not in the AZ) to set up more correctly.
I decided to write up my findings here rather than start a new topic as it seems to fit in with the discussion.
The objective was to cut it to the correct length for resonance, and to measure the impedance and any change to resonant frequency when close to the ground. It will always be used quite low, so I started with the centre at 3m, and the ends at 1m above ground. Wire sufficiently tensioned to keep it fairly sag free.
I had to cut 4m off each end to make it resonant, so that will involve slightly less walking in future!
Using an MFJ analyser, this was the result for 3m centre, 1m high ends:
And this with centre self suspended about 15cm above ground, 1m high ends:
Putting the ends of the dipole close to the ground (eg removing the walking pole end supports) took the resonant frequency well below the range of the analyser (1.7MHz lowest tuning point), such that there was no hint of a dip as I tuned down towards the limit. Changing the centre height made no noticeable difference to this.
With the ends at 1m high, altering the height at the centre made little difference to the electrical characteristics between the test values of 15cm and 3m.
The ground was damp, flat, grass covered earth with a loose rocky substrate based on how the pegs pushed in.
So, on this basis I think in future I will try to keep the ends clear of the ground, and not worry about the centre.
I did connect up the FT817 and call CQ a few times on CW, but the band was very quiet, and I got no spots on RBN - there seemed to be no skimmers in or near UK reporting any signals at all, which is perhaps un-surprising just after mid day.
The test site, looking across to Caradoc G/WB-006
Calling CQ, the feed point is self supporting just above the yellow kite winder. T1 tuner no longer required for top band, I tried it on 80m but it didn’t want to know…:
Looking forward to the next outing but tomorrow, Wednesday 8th March, is loooking a bit too cold for me by dusk. Hoping to be out during the day, but not on top band!
73
Adrian
G4AZS