|John Lemay G4ZTR builds SCQM70UC 70 element 1296MHz yagi! Full story coming soon...||WS48179 built by David Ross GI4SNA now on the side of his tower for the 70MHz UKAC contests!|
Build date 11-12-2017... Visitors Online: Visitors Today:
All of my VHF/UHF low Q yagi designs have been moved to the PowAbeam Antennas website!
Q-factor can have a major impact on stability in bad weather and proximity to other structures. It's not just about available VSWR bandwidth. The lower 'Q' designs offer greater stability, and should be considered as the first choice in locations suffering extreme climatic conditions, perhaps even above other desirables!
Average Q-factor in the real world: A guide based on field trials (during my six year research project) and recent analysis (2014)...
The simple rule, the lower the 'Q' the better is stability. Yagi designs sporting a very steep exponential curve when plotted are not ideal, this dramatically raises the average 'Q'! Why Low Q?
I have been brutal in selecting the final designs. As an example, the criteria for selection at 144MHz is based on average Q-factor <25, and lowest antenna temperature seen in TANT for both G/Ta @ 30° and G/Ta @ 90° elevation.
My research revealed that yagis designed this way (as in my selection criteria detailed above) are more likely to deliver performance shown in software than any other designs out there! The killer in any yagi design is high levels of reactance and/or high Q!
Designed in K6STI's professional version software my presentation here uses the free, powerful and unique 4nec2 by Arie Voors! Good designs stand up to examination and scrutiny in any software package!
Dimensions and stacking distances can be found in the .nec files which are the latest and most up to date versions. They are the actual design files so accuracy is assured!
A conductive boom shortens the electrical length of an element that is close or passes through it!
ALL DIMENSIONS SHOWN IN MY ANTENNA FILES ARE FREE-SPACE
ELEMENTS NEED TO BE INSULATED FROM AND RAISED SUFFICIENTLY HIGH ABOVE 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. However, on the higher bands (432MHz and up) even well designed high quality element mountings exhibit some boom effect which may require correction!
Don't use plastic end caps on the parasitic elements unless specifically directed to do so, otherwise it can cause problems with the tuning. End caps can be used to deliberately lengthen elements!
Element diameters/sizes (check .nec files), typically they are as follows:
|Stauff Clamp||Badland Element Clip Here||PowAbeam ParAclip Here|
|Ideal for the lower bands, no boom correction at 50/70MHz. Correction required above 100MHz! Two bolt fixing, holds 1/2-5/8 Inch elements etc.||Ideal for the lower bands, no boom correction at 50/70MHz. Correction required above 100MHz! Single bolt fixing, holds 1/2 Inch elements.|
144MHz Parasitics = +6mm
432MHz Parasitics = +7mm
Driven Element = No Correction
|G4CQM design for higher bands, no boom correction required at 144MHz. Minimal correction at 432MHz, in practice makes little or no difference! Single bolt fixing, holds 3/16 Inch rod!|
Keeping individual lead lengths as short as possible when making connections to the driven element is so important on the higher bands (144MHz and up). Stray feedpoint reactance can degrade SWR and increase mismatch loss. It can also account for VNA plot differences when comparing identical antennas, due to slight variance in construction!
When stacking horizontally polarized yagis one above the other then mutual coupling is greatest, this can impact on matching and may require some adjustment. Stacking yagis side by side reduces coupling. All of this will depend on a particular design, some yagis are less affected than others, low Q designs most resilient.
Warning: As a result of mutual coupling there may be a small difference required of position and length for the Driven Element when in four yagi group. Check .nec files to see if this applies!
Optimal stacking depends on yagi aperture and sidelobe content. I have used DL6WU, or 0.95 of, or 0.9 of, dependant on design for best overall results. DL6WU based Stacking Calculator!
Some commercial websites make highly questionable statements:
1. We do not use "amateur" programs like NEC4, NEC2...!
These are not "amateur" programs...
NEC is the reference-accuracy Numerical Electromagnetics Code (Method of Moments) from the United States Lawrence Livermore National Laboratory.
NEC is a sophisticated and powerful program for electromagnetic-field analysis. It evolved from mainframe computer programs written in the 1970s. Gerald Burke at the Lawrence Livermore National Laboratory has since brought NEC to its present form. NEC-2 is the latest release authorized for unrestricted distribution. Professionals and academics use NEC for a variety of electromagnetics problems, including antenna analysis. They have thoroughly characterized the code and validated it against measurement.
2. Using EZNEC Pro/6 with the NEC4/2 engine means accuracy is as good as it gets and 30 years worth of bugs do not remains as they do in NEC2 based applications...!
There are no bugs in NEC...
NEC-3 is a modified version of NEC-2 designed to include a Sommerfeld model to properly model wires buried in or close to the ground...
NEC-4 is an enhanced version of NEC-3 designed to better model very small antennas, including those on cellular phones and WiFi routers. The most recent release, 4.2 also includes a better version of the Sommerfeld model used in NEC-3 for in- and near-ground wires. Also added current sources instead of just voltage sources as in previous models, and uses a new memory management system allowing arbitrarily large designs. NEC-4 remains proprietary with the Lawrence Livermore National Laboratory including the University of California, and requires a license.
This new website consumes much less Internet bandwidth and presents only the supporting information...
Derek Hilleard G4CQM
|WS715446 & WS28162 Low Q Yagis from Roger Banks GW4WND of the DXShop ideal for UKAC!||4 x WS718562 Yagis built and operated by Richard Mason G6HKS, makes a super EME array!|
|PowAbeam Antennas professionally made by Roger Banks GW4WND of the DXShop...|
Round booms (2M & up), sealed DE molding, unique ParAclips for no compromise in performance!
My commercial involvement ended in 2013, it was then that I handed over to Richard Mason G6HKS and also Roger Banks GW4WND of the DXShop ownership and production of PowAbeam Antennas. So, please contact Richard for kits & parts and Roger for ready made yagis! Make sure you send them a link of the actual design you are interested in. Please note that Richard's website contains important build information!
Also check out the page of Sven SM7DTT who stocks a comprehensive range of Antenna-Accessories including suitable ferrite and test equipment!
|50MHz WA65055 built by Jerry Ward G4JQN!||70MHz WA45055 built by Richard G4WFR!||432MHz WA718562 built by Richard G4WFR!|
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