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Derek Hilleard G4CQM presents: All weather performance VHF yagi antenna designs...
Unique 2M Band (144MHz) 50Ω Direct Feed Low Q Low Noise yagis. Designed and built by G4CQM, each design featuring fat chunky elements!
AGTC_lite was progammed by F5FOD with help from DG7YBN. In my opinion this is an excellent replacement for TANT and offers several advantages. I have tested AGTC lite running in Windows Vista (32 bit), Windows 10 (64 bit), Ubuntu Mate 16.04 LTS/18.04 LTS (64 bit), and Openindiana Hipster (64 bit). In a Windows environment it runs directly. Using Linux and Unix you will need Wine installed. Seen above is an animation I have created to demonstrate AGTC lite whilst running and displaying one of my designs.
Aluminium for these projects can be obtained from the following suppliers. In the UK we are still using imperial sizes. When ordering aluminium for the elements specify 6063-T832 or equivalent, best for conductivity! For long booms over 4M in length use heavy wall gauge. Links open in a new window:
1. ABS Metals
3. Blackburns formerly Baco Metal Centres
5. Durbin Metal Industries Ltd
6. Forward Metals Ltd
7. L.A. METALS LTD
8. PR METALS
9. Smiths Metal Centres
Derek G4CQM describes two easy methods that make Direct Feed using a simple Split Dipole not only feasible but also work very effectively...
|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!
|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, and with any computer operating system. I have used Symbolic Dimensions/Expressions to describe element diameter, position and length. Stacking distances are shown at the top of each four yagi file against Vertical and Horizontal distance headings.
All of my designs use a simple Split Dipole as the Driven Element, it's so easy to make and very efficient!
Keeping individual lead lengths as short as possible when making connections to the Split Dipole is most 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!
G4CQM's Dipoles Explored blows a hole wide open in myths propagated on the Internet concerning the pros and cons of particular driven elements!...
|Split Dipole||Folded Dipole||LFA Loop||V Split Dipole|
Few Radio Amateurs have any substantive idea of the system noise temperature for their receiving systems, and many wrongly believe that they can learn their stations G/T figure from tables such as the VE7BQH yagi comparisons or manufacturers individual antenna specifications!
On the other hand tools such as AGTC LITE can help our understanding by looking at overall pattern cleanliness and quantifying this as a measure of antenna temperature (T_ant) which includes internal losses expressed in numerical value degrees Kelvin. By noting the full range of antenna temperatures from 0 - 90 degrees elevation puts this into perspective and aids meaningful comparison, yagi Vs yagi.
As an example: If you have set T_sky to 200°K then at 90° elevation angle the nearer your yagis temperature (T_total) is compared to the sky temperature then the better it is!
Unfortunately man-made noise in urban environments may not be emanating from one direction alone, but all around instead, and most likely in differing magnitudes on different beam headings!
However, a 'Low Noise' yagi design can help in copying weaker signals by reducing the overall noise pickup. If attention is paid by the designer to delivering a cleaner pattern in both planes (E & H) then noticeable and real improvements can be observed!
All of my designs benefit from careful side and rear lobe reduction, and in both planes (E & H)!
Yagis exhibit a frequency response similar to that of a Low Pass Filter (LPF), it is most important to ensure that the cut-off frequency is well above the desired upper bandwidth limit!
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!
Don't confuse true resonance with matching, the point of lowest Q and minimun VSWR are not always on the same frequency! A good match on a spot frequency can be effected no matter where Q is, but the available (useable) VSWR bandwidth around that spot frequency reduces as average Q-factor rises.
Genuine low Q yagi designs are more likely to deliver performance shown in software models because they are less sensitive to their surroundings!
You cannot assume a yagi to be low Q just because the VSWR bandwidth looks good, fatal mistake!...
|Competitor (A) SWR looks great!|
|Competitor (A) Q-factor not so good!|
Average Q-factor in the real world: 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 an exponential curve (very steep average Q-factor plot) are not ideal. This dramatically raises the average 'Q' and warns that the cut-off frequency is too close or may already be in band! Dielectric loading effects caused by wet weather will drag the cut-off frequency even further LF and give rise to instability!
DL6WU yagis exhibit an ideal average Q-factor gradient, designs with an exponential curve are at risk!...
|DL6WU ideal gradient!||Competitor (B) design at risk!||Competitor (C) design worse!|
|CQM9DX SWR looks great!|
|CQM9DX Q-factor looks great too!|
|Badland Element Clip|
|Badland Element Clip|
|PowAbeam DE moulding here||Badland element clips here|
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 recommended distances!
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.
Also see: Diamond or Conventional yagi four stack?
DL6WU based stacking calculator, simply enter the -3dB Beam Width (Degrees)...
G4CQM's 144MHz Yagi Table... Information based on Interactive Table published by Lionel Edwards (VE7BQH) Issue 105, Oct 2, 2017...
G4CQM designs have been included for comparison purposes...
This particular interactive table has been created using Gnumeric, a free and lightweight yet powerful (open-source) spreadsheet program! For Microsoft Windows users I strongly advise using Gnumeric Portable...
Download... G4CQM_144_TABLE.gnumeric (Filesize only 50482 bytes!)
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