Aug 9, 2008

Simple HF multiband vertical

Your own HF multiband vertical

During my first participation in the annual HB0-PI4TUE mountain dx-peditions back in 2003, I found that by using a fishing pole and some wire, you could build a very simple quarter wave vertical. Just tape a vertical wire along some fibre pole, throw a few wire radials in the bushes and go for it. Adding a second vertical wire in parallel made it a multiband antenna. This newcomer's antenna was a bit mistrusted at first, against all other vertical aluminium on the premises.
But it soon proved very competetive. The other commercial trapped vertical, showed not only signal loss, but were more noisy on reception. From then on everyone wanted to use this 'new kid on the block' and the 'trapper' soon became the 'lone black sheep'.


From that summer onwards we started playing with the concept at thome and during field day set-ups. We wanted more bands along a single fibre rod, so why not make a 10-12-15-17-20mtr vertical using 5 vertical wires?
In order to avoid coupling between the radiators we used spreaders to keep them separated from the main rod. Many attempts and numerous antenna measurements using various analysers proved; mutual coupling is-a-big-deal. When we thought we finally had concord the animal we took our new 5-band design to the mountain the next summer. Up there it took 2 days to get the thing working..... :(

Some don't s on multiband verticals:
- anything closer spaced than about 30cm (1 foot) is prone to cause heavy coupling
- too many bands will not work; forget about 40-30-20-17-15-12-10
- not using a balun/common mode filter makes the coax just another disturbing radiator
- elevated radials double the trouble of coupling and adjustments
- do not expect that the ground radials act similar in another location
- do not expect that it will be reliable and still have the same SWR after some rain and/or wind

If you have plenty of time and patience, a lot is possible. You will notice that for a single frequency it will 'jump between radiators' and the same goes even more for elevated radials. You have to connect/disconnect wires every time to verify which radiator/radial is dominant. For a field day style setup another approach is required. After trying many designs, we now have some reliable repeatable ground rules.


Basics of the multiband quarter wave wire-vertical


Note: this is a tribander for 20-30-40mtrs



Vertical: 12mtr/40ft fibre pole
Wire: anything will do, start with a length just over 0.25 lambda
Spreaders: plastic about 60cm/2ft long. Used on both top and bottom
Radials: minimal 8 laid on the ground, either straight or in zig-zag shape
1:1 Balun: use any decent common mode choke (not in drawing)
Adjustment: start from the lowest frequency upwards

Since the rocky bottom and many plants/bushes on our mountain do not allow for many straight radials, we opted for 8 radials laid out in zig-zag shape. Each corner was fixed uding an iron nail. This created plenty coverage of copper nearby the radiator and is accepted to perform better as fewer longer radials. See also "Lowband DXíng" by John Devoldere ON4UN, or "Monopolantennen und Vertikalantennen" by Gerd Janzen.



First vertical; 12mtr/40ft fibre rod



Above vertical resembles the design in the drawing. It has 3 radiators for 20-30-40mtr band
A current balun was used with a toroidal core type FT240-K.
A 1mtr length copper tube was hammered down and served as ground radial connection.

BONUS:
- the 30mtr radiator length is close to 0.75 wavelength on 10mtrs.
- the 40mtr radiator length is exact 1.5 wavelength on 15m.
Both showed a good SWR pattern. It performed very well on 10 and 15m where the typical high angle lobes proved worthy during the rare european Es openings .



Second vertical; 6mtr/20ft fibre rod

This uses the same basic design, but has 5 radiators





Above vertical has radiators for 10-12-15-17-20mtr band.
The current balun uses a toroidal core type FT240-67.
A 1mtr length copper tube was hammered down and served as ground radial connection.
The small blue box is the clue for a former blog entry :)


Performance
Both antennas were pretty quickly setup. The minor adjustment changes were due to different ground environment from home.

Using these two multiband antennas we have made some 5500 contacts. They both perfomed very well. During the IARU contest we managed to make >800 qso's in an 18hour effort.
Operators found the antenna very competeteive on 40mtrs where we could run a frequency without problems every time we wanted.

Think about it; your domestic situation certainly allows for any this kind of antenna.
And its a full size performer, not to mention cheap !


6 comments:

Mark, PA5MW said...

OK, just found a typo in my text:
The 40mtr radiator in antenna 1 is also 0.75 wavelength for 15mtrs. And not 1.5 wavelength as advertised.
Oh well..

ON3JT Joost said...

I use the same principle on my roof. Works great, 5 bands without loading coils, traps or anything else...

Dave Hartman said...

Don't know if this already posted or not:
AC2GL - Dave
Looks like half of a fan dipole stood on end.

Mark, PA5MW said...

Correct Dave.
The intitial design was found in Rothammels Book.
I believe High-Gain is using a similar principle for its HY-TOWER JR, 10/15/20/40/80M
73, Mark

Steve Donegan said...

I am curious if a dual band, 10M and 6M can be fabricated with old CB radio whip antenna elements cut to size - a feed to both at the base and 3-4 inches separation - your thoughts?

Mark, PA5MW said...

Yes that is possible.

look at below example; 10-12-15-27-20m multiband vertical, all made from old 27Mhz CB antenna parts: http://monitorstation.ccms-best.nl/about-us.html

But maybe you need just a tad bit more than 3-4 inches separation. The closer, the more critical adjustment becomes. However at only two bands (10 and 6m) which are non-harmonic resonant bands, this should be easy.