Aug 24, 2008

Simple HF Multiband Vertical ; Part II, the home Stealth version

NCDXF HF Beacon Monitoring

For several years now I'm using VE3SUN's computer program "Beacon Observer".
This is a very informing tool to monitor the NCDXF HF beacons. I like it better than Faros.
Via RS232 it controls your receiver, switching between all upper 5 HF bands from 20 to 10mtrs.
In 3 minutes you have a graphical overview of all world-wide 18 NCDXF HF beacons per band, which each operate in a timed sequence for 10 sec. 18 beacons * 10s * 5 bands= 15minutes for a total overview.
This kind of monitoring shows band propagation 24/7 and during the year the several seasonal propagation changes which are huge.
There are numerous band openings which happen when we are away from our shack. Ever seen the typical Es night openings on 10mtrs? Well there have been many this summer! After an evening Es opening, usually every 3-4 hours later there's are more. That also goes for 50Mhz and was proven this summer during our HB0-holiday trip as the only USA qso's I have made on 6m where at 01:30 local time....
Anyway, I will spend another blog about my NCDXF HF monitoring system in more detail in the near future.

Stealth antenna

Receiving these HF beacons requires a multiband omnidirectional antenna.
In order to minimize the visibility of any 'vertical hardware' I have built a multiband fibre glass rod vertical according the earlier written basics and put it between the branches of a near-dead tree at the boundary of my garden. Some non-glossy black and green spray paint has made it unobtrusive for non-hams.
However 3 issues made it impossible to tune it correctly for all 5 bands:the heavy coupling between radiators, elevated radials which doubled the adjustment issues and the accessibility in the tree.
Now, you could suggest to make a temporary setup in the clear in the middle of the garden so you can lower the antenna quick&easy for adjustments. Mind you, this is not a true (on-) 'groundplane', but 'elevated parallel radiators and radials' which makes tuning a critical task. And it behaves different when you move its location.
For RX-only good SWR is not a requirement, it just needs to receive well.
My design has evolved in an MK-II version over time. Its MTBF was pretty low. Some radiators got tangled up due to heavy wind. The radials got accidently cut while gardening. Next the cheap&easy connection hardware corroded, despite anti-corrosion spray and stuff.

A good reason for building a newer, even better version every 1-2 years. After all; there's nothing like building a new antenna :)

Can you spot the antenna?

The MK III version
The most important lesson learned during building our multiband verticals in the field is that the vertical radiators need enough space between them to avoid those coupling effects. Especially at the bottom/feeding point.
Some specifications of this new version:
  • Radiators are spaced 30cm from the central fibreglass rod
  • 5 radiators adjusted for 14.100/18.110/21.150/24.930/28.200 Mhz
  • 0.5mm, non-stranded, tinned, isolated hookup-wire (Conrad #211169)
  • Current balun; 7 bifilar windings on F-150A-K core
  • Electrical connections twisted and soldered
  • All heavily coated by multi layers of vulcanizing tape
  • Some more non-glossy black & green paint spray

Using my Palstar Antenna analyzer and its supplied 1:1 balun
, the adjustment went pretty well. Radials had to be disconnected often, to prove if they behaved as wanted.

How does it perform ?

Slightly more HF output, but more important: it is constant over the 5 bands. I'm not using the pre-amp on any band any more now, the (earlier modified) TS570SG shows enough sensitivity.

Any wishes left?

No. Let's wait and see for a few years before I start thinking of MK III.....

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