Sep 14, 2008

Top loading on 7Mhz vertical

Just to kill a free sunday afternoon we decided to do some experiments with toploading on a vertical in the backyard of a local ham.
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.

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 ?

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...

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".


Time for some coffee :))


acolla said...

Hmmmmm. Did it resonate on 7020? Or just the impedance was ok (50 ohms) there? How many ground radials did you installed?
I am playing with a 40m vertical as well, but I've installed a antenna tuner on its base (Icom AT-120), with 16 ground radials. Now I will try to add some capacitance hat, so the current moves up, instead of heating the ground...

PS: I lived in Nederland (Amterdam and Roterdam) from 1990 to 1991. I miss it a lot.

Tot ziens!

Mark, PA5MW said...

Yes, we make use of an antenna analyzer and an HP network analyzer when doing stuff like this. Resonance was noted at Xc=0.
Ground 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 !

acolla said...

Thanks for the answer!

Andy - PY1VHF