We wanted to acquire some practical experience on this subject in case we will need this for any future fieldday style setup.
Starting point: a 7.025 Mhz vertical 0.25 length, downsized to 65%
Using 2.5mm2 copper wire (about AWG13), a vertical wire was taped to a 6.5mtr (21ft)fibre glass rod. A top-hat was fabricated from the same copper wire, soldering 8 spokes on a round piece of circuit board.
Result: 8 spokes 0.55m => 7.95 MHz
Next a series coil was installed at 1m70 height (5.5ft) to lower the resonance to the wanted CW band portion (7.025Mhz). This required 12 windings with about 7cm diameter.
Going from 8 to 16 spokes
Not being satisfied with the series coil we removed it and decided to start enlarging the top capacitance loading.
Result: 16 spokes 0.55m => 7.41 MHz
That's a 7% change, more than 500Khz. Nice but not there yet.
Adding a ring on the top hat
A single ring was soldered on the 16 spokes. This created a far more rigid construction.
Result: + ring => 7.35 Mhz
Hmm...bit of a dissapointment here.
There's no room for additional spokes on the copper mounting plate so let's try lengthening the spokes.
16 spokes + ring + 0.3m extra length on just 4 spokes
Enough bare copper wire available so let's "solder-glue" some extra length on just 4 spokes.
Result: 0.3m extra length on 4 spokes => 7.06 Mhz
Almost 300Khz down now. Close, but no cigar yet !
16 spokes + ring + 0.15m extra length on 8 spokes
The 4 spokes look silly, right?
Now how about lengthening 8 spokes with just 0.15m? Will 8x 0.15m be the same as 4x 0.3m ?
Result: 0.15m extra length on 8 spokes => 7.12 Mhz
Nope; this is a small step backwards. But at least we get confirmed what works best.
Final option: 16 spokes + ring + 0.15m extra length on 12 spokes
OK, just add a few more extra lengths...
Result: 0.15m extra length on 12 spokes => 7.02 Mhz
Bandwidth is 350 KHz at SWR 1,5
*Ding* we're finally there.........
Oh .. wait his XYL comes home and looks up in the sky: "What a nice sun-flower you boys have created".
Eeehhrm.......???
Time for some coffee :))
Yes, we make use of an antenna analyzer and an HP network analyzer when doing stuff like this. Resonance was noted at Xc=0.
ReplyDeleteGround consists of a 3mtr long 22mm dia copper pipe and several buried radials. Don't know how many, but they are layed out in Z-shapes to cover as much area as possible.
Z, Rs, Rground and the resulting efficiency is a different story. Maybe we will perform some additional field strength measurements to see what the difference is between f.e. full size 0.25 radiator, the small tophat +series coil and the final version.
Groeten vanuit Brabant !