G4CQM's Antennas


WS8C9 8 ele 144MHz special build by Roger Banks GW4WND of The DXShop

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Balun... Derek G4CQM describes two easy methods that make Direct Feed using a simple Split Dipole not only feasible but also work very effectively...

1. Nickel-Zinc (NiZn) ferrite optimised for operation at VHF frequencies!

On the Short Wave bands your coaxial cable feeder length may be similar to the λ in question or a few multiples thereof. Under certain conditions the screen itself can display a degree of resonance. It is likely therefore that the use of a balun may be required and of benefit in eliminating radiation from the feeder and prevent it from becoming part of the aerial.

However, on the VHF bands a balun may not be required and in fact introduce unnecessary and additional losses. The most important consideration with any yagi beam is that the driven element is tuned and matched correctly. With a real 50Ω yagi this is very easy to achieve directly during the design process. Meanwhile keeping reactance low at the band edges particularly the HF end is paramount for proximity and bad weather stability.

If you are in any doubt a simple solution is the use of suitable slip on or clip over ferrite placed close up to the feedpoint. This will impede the flow of RF current on the outer surface of the screen.

Clip on ferrite sold in many electronic outlets and mail order catalogues may not be ideal. Most are made from Manganese-Zinc or Iron powder and have little effect at VHF frequencies (100MHz or higher). Instead using Nickel-Zinc (NiZn) ferrite optimised for operation at VHF frequencies is a far better choice!

Nickel-Zinc (NiZn) ferrite optimised for operation at VHF frequencies!
Nickel-Zinc (NiZn) Z Vs Frequency!
2. The λ/4 Half-Loop Trick!

Back in 1976 I attended an international electronics exhibition in Paris and had a meeting with the famous Mark Tonna F9FT. Mark showed me a very simple trick avoiding the need for a balun or ferrite on your 144MHz yagi feeder!

On the 2M band simply measure λ/4 (no Vf correction) from the DE, mark this point with red tape. Push feeder toward DE creating a half-loop and tape red marker point to boom. Continue securing feeder along side of boom as normal.

Seen opposite λ/4 Half-Loop Trick used on CQM10DX!

NB The λ/4 half-loop trick will only work on a metal boom!

λ/4 Half-Loop Trick used on CQM10DX!

Build Dimensions... can be found in the .nec files which can be opened/viewed with your web browser or text editor (Crimson Editor, Pluma etc), and with any computer operating system. I have used Symbolic Dimensions/Expressions to describe element diameter, position and length. See example .nec file opened in Firefox web browser...

.nec files can be opened/viewed with your web browser!

PADIA = Parasitic element diameter
DEDIA = Driven element diameter
RP, DEP, D1P etc = Element position
R, DE, D1 etc = Element length

Stacking Distances... are shown at the top of each four yagi .nec file against Vertical and Horizontal headings.


Construction Guide...


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