Low Q Long Yagis by G4CQM for Terrestrial VHF/UHF DXing

26-08-2023... In past years wide spaced Long Yagis were the number one choice for serious Terrestrial VHF/UHF DXing. Those old designs created by adopting basic principles along with trial and error including a generous testing regime. The problem, they were mostly narrow bandwidth gain optimized designs subject to wet weather and close proximity effects, so impacting on overall performance! To a certain degree mitigation could be effected by using a folded dipole (difficult to homebrew) driven element.

Derek G4CQM presents designs specifically for Terrestrial VHF/UHF DXing, viable and simple split dipole solutions. A selection of modernized Low Q wide spaced Long Yagi (LY) designs optimized with K6STI's YO7 Professional version software and calibrated with NEC-2. Limitations associated with NEC-2 have been carefully avoided as there are no tapered (no junctions) or bent elements, and no close proximity spacings. Unique element mounting clips designed by Derek avoid the need for correction by conforming to the DL6WU mounting directive. This requires that each element is raised more than the boom radius (r) above boom. A special tuning technique conceived by Derek gives rise to a low average Q-factor with subsequent wider bandwidth than older traditional designs, critical in delivering a stable antenna. From experience the very best designs occur when YO7's unique algorithm and NEC-2 concur.

Advantages with these new designs include no matching, very easy to build, lightweight/low element density, with reliable performance. Finally, don't confuse these new yagis with the OWA (Optimized Wideband Antenna) concept which requires very close spacing between the DE (Driven Element) and D1.

For parts to build these and many other CQM designs contact G6HKS Yagi Kits & Parts for full details, specifications and prices. Just to be clear regarding designs detailed on this page, the DE requires 5/8th Inch (15.875mm) OD tube, whilst the parasitic elements are all 3/16th Inch (4.7625mm) OD rod.

Old Vs New - Average Q-factor comparison and available bandwidth

These graphs which compare old style 2M7N50 against the new 2M7N50LY design clearly demonstrate the relationship between average Q-factor and available bandwidth. Yagis have a frequency response similar to that of a low pass filter and so it is of paramount importance to keep the cut off frequency out of and above the desired passband. The prospect of high stability far outweighs any slight loss in forward gain when comparing designs, and the new low Q yagis offer a much greater degree of resilience when deployed in the real world! Software design limitations and construction tolerances are easily absorbed in the new low Q designs without any noticeable loss in overall performance.

Understanding YO7

To reduce screen clutter, YO7 doesn't label the figures displayed within the Yagi patterns. They are shown in yellow font, 2M7N50LY example above.

YO displays elevation angle for Yagis over ground and gain figure-of-merit for single-Yagi, free-space models.

YO defaults to a generalized definition of front-to-back ratio.

The notation 12.7-j15.4 means a resistance of 12.7 ohms in series with a reactance of -15.4 ohms.

Z stands for impedance.

The lambda symbol (λ) means wavelengths.

YO uses a generalized notion of standard front-to-back power ratio to characterize pattern quality. Conventional F/B is the ratio of forward power (at 0 degrees) to that radiated in the opposite direction (at 180 degrees).

YO's generalized F/B is the ratio of forward power to that radiated within a specified region to the rear of the antenna. This is called front-to-rear ratio (F/R).

Yagi designs maximizing conventional F/B often have large backlobes at angles other than 180 degrees. Much better patterns result when you optimize a Yagi for F/R.

The F/R region begins at 180 degrees and extends forward to a specified angle (90 degrees by default).

YO is calibrated to NEC, the reference-accuracy Numerical Electromagnetics Code from the Lawrence Livermore National Laboratory. Invoke NEC from within YO by pressing the spacebar. Letters NEC shown in the top right hand corner of results screen confirm the process has concluded.

Finally... Element lengths shown in the tables are half lengths.

Please carefully read the following notes... If you haven't built your own yagi before, hopefully, construction guidance offered here will stop you making mistakes that will prevent your build from working as it should... These designs don't require any user adjustment!

• Boom size
  
  All featured designs require 3/4 Inch (19.05mm) square section aluminium, and use heavy wall gauge for strength. However, some form of overhead or underside bracing is required for the very longest booms.

• Element size
  
  Parasitic elements require 3/16 Inch (4.7625mm) diameter solid rod whilst the DE 5/8 Inch (15.875mm) is tube.

• Measuring
  
  Lay out boom, mark position of reflector element, measure all other element positions from this point.
  
  Aim for the nearest mm!

• Tolerances
  
  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 desired 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!

• Boom Correction
  
  Specialist element mounting clips designed by G4CQM available from G6HKS Yagi Kits & Parts don't require correction!

• End Caps
  
  Avoid using plastic, vinyl, or rubber end caps on any of the elements (reflector, driven and directors), these will affect tuning and make wet weather performance worse! Seal the DE with flush mounting end plugs.

• Driven Element
  
  The DE length is end to end which must include the centre gap of 15mm!
  
  Example 2M7N50LY - From the YO7 elements table the driven element total length is 481.5mm x 2 = 963mm. To achieve this in practice deduct the center gap dimension of 15mm from the total, and cut a length of tube to 948mm. Finally, cut this into two pieces, each will then be 474mm in length!

• Boom length
  
  Allow an additional +30mm at either end of the boom to maintain tuning across its entire length, and also give a professional look...
  Example: 2M7N50LY Reflector to D5 spacing is 3478mm + 30mm + 30mm = 3538mm BOOM LENGTH!

• VSWR
  
  It is unlikely that reflected power will be exactly 1:1 in the operational bandwidth, a small amount of reflected power is normal due to software limitations and construction tolerances... Don't tune with your test equipment, use as a guide/reference only!

• WARNINGS
  
  DOUBLE CHECK YOUR MEASUREMENTS BEFORE YOU CUT & DRILL!
  
  DON'T ALTER THE DESIGN IN ANY WAY I.E. NUMBER OF ELEMENTS, LENGTHS, DIAMETERS OR POSITIONS. IF BUILT CORRECTLY THESE DESIGNS WON'T REQUIRE ANY ADJUSTMENT!

When founding PowAbeam Antennas way back in early 1994 I soon realised that unique and professional looking components would set my designs apart from the competition. Working as I did in the Telecommunications and Radio Communications industry I was fortunate to have access and be able to design the Driven Element moulding and specialist parasitic element mounting clips.

Legal

• THESE YAGI ANTENNA DESIGNS AREN'T 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.

• MY WEBSITE DOES NOT USE COOKIES, EXTERNAL LINKS MIGHT!

More web pages/articles from G4CQM

• Antenna Designer UK - 'Specialist Yagi Designs' by G4CQM

• 'Yagi Antenna Design' by G4CQM