Sep 21, 2008

Getting some of those 'small projects' done- Part II

With the local weather being such good during the evenings, I managed to do a lot this week:

Entry panel
  • Added a second coax relay
  • Mounted DCI 145-2H 4-pole cavity bandfilter
  • All wired & tested

I'm now able to switch 3 VHF antennas (stack of 2x 4el horizontal, 4 el vertical yagi, vertical omni) to 2 different multi-mode VHF transceivers. The DCI bandfilter can be switched in on the omni-directional vertical. It is for experimental purposes only.
This filter was used at my old QTH were the IC-746 suffered from heavy IMD (receiver is wide open from 60-200MHz). A local low power 100.7 MHz FM BC station always was audible in the background throughout the whole 2mtr band, in AM modulation :). Next there were several near-by hams which added up to the to total 'distortion' picture.
At my current location I'm only using multi-mode transceivers which have decent bandfiltering in their front-ends, starting before the internal pre-amp. There seem to be no strong local signals, both in and out-band here at my new QTH. After several days of testing I simply cannot detect any difference with or without the filter, besides the small extra loss.

Exchange the rotor of the FM broadcast antenna setup
The old Channel Master 3-wire controlled rotor (1970's) was removed. It was never in sync with the controller, which in turn also got stuck on several occasions.
From a local ham I bought a Yaesu G-250 in mint shape. This is exactly what is needed; a simple light-weight reliable rotor for a small antenna setup. One minor issue; the brackets can only accomodate 38mm max. The current mast is 50mm and stuck forever on its mounting plate. Now being a ham for some years, you have lotsa alumn lying around right? I cut a piece from a 32mm tube and pop-riveted that to the 50mm mast on the roof. The FM BC antenna is build from 40mm PVC so no worries there.

This certainly looks much better than the ancient 70's rotor (see previous blog 29th april).
Black 'Body Protection' car-paint was used to weather proof some parts and darken shiny obtrusive parts.

New 1:1 balun for the 160m 'reference vertical'
This was on the list for some time; check the resonance adjustment of this reference antenna (see blog of 18th january), change the balun, clean and weather proof the connection box. The annual check-up. The old balun was a quick&dirty home made version.
The new balun was made from 2 binocular ferrite type #73, glued together and fixed with shrinking tube.
Two sets of each 3 turns teflon coated wire. One set was packed in an extra separate teflon tube.

In the connection box you see in the left upper corner the series resonance coil on a FT114-61 ferrite core.
Each winding shifts resonance about 25KHz. Resonance was set at 1825. Bandwidth is about 25-30KHz (SWR=2). Antenna impedance at Xs=0 still remains at 50 Ohms, which shows my ground system is not sufficient. Bad luck, my garden just not allows for many radials of any decent length. I'll have to stick with the copper 3mtr 2mm dia ground rod and 4x 4mtr length buried radials.
There have not been any verification tests, besides the usual on 160m and the upper part MW BC. Theoratically the new balun should do be a better job in common mode rejection. In practice it surely looks 599.

The winter low-band season has already started.

Now what about:
- that phased vertical system?
- a 'TX' antenna for a change?

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 :))