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!
|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!
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...
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.
- Boom size
All featured designs require 1 Inch (25.4mm) square section aluminium when using element clips from RW BADLAND LTD!
- Element size
Parasitic elements require 1/2 Inch (12.7mm) diameter tube. Note that the DE is 5/8 Inch (15.875mm) diameter!
Lay out boom, mark position of reflector element, measure all other element positions from this point.
Aim for the nearest mm!
Measure and cut the parasitic elements to the nearest mm. However, it is better to cut them 1mm too short than 1mm too long. Reason: Yagis have a frequency response similar to a Low Pass Filter so it is very important to keep the cut off frequency out of and above the pass band.
- Element Mounting
ALL DIMENSIONS ARE FREE-SPACE
ELEMENTS NEED TO BE INSULATED FROM AND RAISED SUFFICIENTLY HIGH ABOVE THE BOOM TO AVOID BOOM CORRECTION AS PER DL6WU MOUNTING DIRECTIVE! I.E. RAISED MORE THAN THE BOOM RADIUS (r) ABOVE THE BOOM...
No boom correction is required if you follow this advice!
- Element lengths
Double the element lengths shown in the .nec files to calculate the end to end length!
- Boom Correction
USING PROFESSIONAL ELEMENT CLIPS FROM RW BADLAND LTD BOOM CORRECTION IS REQUIRED AS FOLLOWS: ADD TO THE TOTAL LENGTH (END TO END) OF EACH PARASITIC ELEMENT (NOT DE)...
50 & 70MHz = 0mm
144MHz = +6mm
432MHz = +7mm
- End Caps...
Do not use plastic end caps on the parasitic elements (reflector and directors), these will affect tuning!
- Driven Element
The Driven Element length is end to end which includes the centre gap (15mm PowAbeam moulding)! When using PowAbeam moulding on square section boom underside aluminium insert required, specify when ordering!
- Boom length
Allow an additional +50mm at either end of the boom to maintain tuning across its entire length, and also give a professional look. Example: CQM6DX Reflector to D4 spacing is 2400mm + 50mm + 50mm = 2500mm BOOM LENGTH!
DOUBLE CHECK YOUR MEASUREMENTS BEFORE YOU CUT & DRILL!
DO NOT ALTER THE DESIGN IN ANY WAY I.E. NUMBER OF ELEMENTS, LENGTHS, DIAMETERS OR POSITIONS. IF BUILT CORRECTLY THESE DESIGNS WILL NOT REQUIRE ANY ADJUSTMENT!
- THESE YAGI ANTENNA DESIGNS ARE NOT FOR COMMERCIAL USE OF ANY KIND WITHOUT PRIOR CONSENT!
- Private experimenters are free to use these designs for their own personal enlightenment. However, no guarantee or warranty is offered or implied.
- Antique White theme by Derek Hilleard G4CQM - Hand coded with Mousepad (text editor) on Xubuntu - Copyright © 2019 All Rights Reserved
Count started Saturday 12th October 2019