Folded Monopole over an Elevated Ground Plane

 

 

Have you ever wanted to activate a Summit or Park where there were no suitable trees or structures to hang your wire dipole antenna from?  This need was the catalyst for me to build the antenna described below. 

My requirements were:

1)      Self-supporting

2)      Reasonably light weight  (5kg)

3)      Reasonably small in size when collapsed

4)      Portable

5)      Easy to erect by one person

6)      Only 7 metres tall when fully erected

7)      Omni-Directional antenna pattern

8)      50 ohm input impedance unbalanced  

9)      Wide bandwidth

10)   Good RF power handling

11)   No ATU required

12)   VSWR below 1.3:1 over the whole 40M – 6M bands

13)   Reasonably low angle of radiation

14)   Relatively cheap to construct  (Around $220.00)

15)   Robust

16)   Using “off the shelf” parts from hardware and fishing supply stores

The idea of a folded monopole is not new.  SteppIR™ make an excellent, commercially made unit that gave me an idea on how to “home brew” my own interpretation of the antenna.

The antenna consists of:

1)      1 x “Laser Level” tripod

2)      1 x 8 metre squid pole

3)      4 x fibreglass tent pole sections

4)      8 x 10 metre steel tape measures

5)      4 x fishing rod loop guides

6)      2 x fishing reels with quick release mounts

7)      10.5 metres of 2mm stainless steel cable

8)      10.5 metres of Dacron line

9)      1 x home brew aluminium hub mount

10)   1 x SO239 panel mount socket

11)   300mm braided copper strap

12)   4 x Stainless Steel Hose Claps

13)   200mm length of 1mm diameter copper wire

14)   Nuts, Wing Nuts, Bolts, Spring Pins Cable Ties, U Clamps, Tent Pegs etc.

 

 

The antenna is a folded ¼ wave monopole on 40 metres, half folded ¼ wave monopole on 30 metres and a ¼ wave on 20 – 6 metres.

Resonance is achieved by adjusting the length of the wire going around the loop which is supported by the squid pole and spreader arms.  This is done by winding the fishing reels in or out.

Permanent marks made on the Dacron line allow quick antenna length selection which speeds up band changing.

The ground plane radials should ideally be adjusted to the correct length for best performance. 

i.e.  A ¼ wavelength for each band.  In practice, leaving the tape measures fully deployed at 10 metres in length still gives good performance and acceptable VSWR across 40 – 10 metres.

The tape measures need to be shortened to about 1.5 metres to work on 6 metres.

The ends of the tape measures should not be electrically connected to earth in any way.  This allows for the greatest capacitive RF coupling to earth to occur.   Only eight radials are required to achieve a very low ripple in the horizontal radiation pattern using this technique.

Lastly, I use ferrite beads around the RG58 coaxial cable nearest to the feed point to decouple the coax outer from the elevated radials.  I guess this would be best described as a Choke Un-Un.

Excellent DX results have been achieved on all HF bands 40 metres and up.  40 metre performance around VK ZL & YB is remarkably good for such a mechanically short antenna!

I’m happy to answer any questions regarding this antenna design and can be emailed at vk5sfa@wia.org.au

 

Happy activations!

 

73  Steve  VK5SFA

 

 

 

 

 

 

 

 

 

 

Pictures are worth a thousand words and I’m not the world’s best scribe, so here is a pictorial description with bullet points.

 

·       Collapsed view of the antenna without ground plane attached.

 

·       Home brew mounting hub and support tube.

 

 

 

·       Mounting hub with tape measures attached.

 

 

 

·       Photo of supply and take-up reels, earth strap and spiral copper RF coupler.

 

 

 

·       Supply reel and RF coupler.  The coupler is made by winding 200mm of 1mm diameter copper wire around a 2mm drill bit and inserting the stainless steel wire through it for an electrical connection.

 

 

 

·       Tent pole spreader arms through the squid pole with spring pins to hold them in position.

 

 

 

 

 

 

 

 

 

·       Fishing rod loop with Dacron line joined to the stainless steel wire using heat shrink.

 

 

 

·       Fishing rod loop with stainless steel wire.

 

 

 

 

·       Ground plane installed.

 

 

·       Fully extended folded monopole on 40 metres.  (Tape measures not fully deployed)

 

 

 

Ferrite Choke Un-Un.  (Not absolutely essential for good performance)

 

 

 

 

 

 

 

 

 

·       Collapsed for easy transportation.

 

 

 

·       In use on 40 metres at the Morialta Conservation Park.   Tie downs and earth stakes should be used to hold the tripod down in windy conditions.  Three yellow tie downs can be seen here holding the antenna in place.