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Derek G4CQM (formerly G8CIW) celebrates more than 50 years as a licensed Radio Amateur, first active back in early 1969!...
Do you really understand the significance of Lionels new interactive VE7BQH Antenna Tables which now incorporates both rural and residential local noise temperatures, simulating local environments? This is truly a game changer, revealing those yagi antenna designs that really are low noise. You will soon learn that there is much more required in the design process than just enhancing F/B (Front to Back) or F/R (Front to Rear) power ratio in order to achieve the lowest temperature yagi. Instead, looking at overall sidelobe content, in particular the forward lobes, has a major impact on delivering a quiet EME antenna system for the higher bands (144MHz and upward)!
CQMDX designs (Self Build/Homebrew) represent a quantum leap in Derek's development of the AOWA (Advanced OWA) style yagi beam using a simple Split Dipole as the Driven Element! There are no bent elements or different diameters to contend with so construction could not be more straightforward!
CQMDX designs situated in a noisy urban/residential environment outperform LFA's and designs with bent elements in most cases, much lower temperature and better G/Ta! Each design has a relatively shallow Q-factor gradient like that of a highly stable DL6WU long yagi! Meanwhile radiation efficiency >98% in all featured designs ensures best overall performance!
A forensic analysis presentation and detailed build instructions will guide the homebrew Ham Radio constructor toward a successful outcome...
|Marcos PY1MHZ (Brazil) builds 144MHz 14 ele G4CQM yagi...|
WS8C9 AOWA designed by Derek Hilleard G4CQM and built by Roger Banks GW4WND of The DX Shop (PowAbeam Antennas), seen assembled and ready to go at QTH of Don G0RQL!
|WS8C9 x 4 residential performance seen in TANT|
|T_sky = 290 K whilst T_earth = 5400 K|
|H = 2777.0000|
|V = 2215.0000|
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, and does not require DOSBox, unlike TANT! 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.
Below an example (CQM10DX) showing the difference between TANT and AGTC results...
|<<< TANT Vs AGTC >>>|
|T_sky = 290 K whilst T_earth = 5400 K||T_sky = 290 K whilst T_earth = 5400 K|
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. Simmal Ltd (Solutions in aluminium)
10. 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...
|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 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 forensic investigation 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|
|>>> Dipoles Explored <<<|
All of my CQMDX designs benefit from a carefully balanced lobe distribution regime in both E & H planes in order to deliver a quiet antenna system!
Unfortunately man-made noise in urban/residential environments may not be emanating from one direction alone, but all around instead, and most likely in differing magnitudes on different beam headings!
Despite all of this 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 will be observed!
|Residential local noise performance as per new interactive VE7BQH Antenna Tables...|
|4 x CQM6DX residential in AGTC LITE||4 x CQM7DX residential in AGTC LITE|
|4 x CQM8DXZ residential in AGTC LITE||4 x CQM9DX residential in AGTC LITE|
|4 x CQM9DXL residential in AGTC LITE||4 x CQM10DX residential in AGTC LITE|
|4 x CQM12DX residential in AGTC LITE||4 x CQM13DX residential in AGTC LITE|
|4 x CQM14DX residential in AGTC LITE||4 x CQM14DXL residential in AGTC LITE|
|4 x CQM16DX residential in AGTC LITE||4 x CQM7016DX residential in AGTC LITE|
|4 x CQM7018DX residential in AGTC LITE||4 x CQM7022DX residential in AGTC LITE|
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|
UK Antenna Manufacturers for parts, kits, and ready made yagi antennas...
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)...
Download link... for the latest official VE7BQH Antenna Tables.gnumeric (only 152,040 bytes!)
Gnumeric Spreadsheet is an open-source spreadsheet program. It is available cross-platform, for Windows, Linux, and UNIX operating systems. The .gnumeric file format is compressed and allows download of the VE7BQH Antenna Tables in a smaller file size than even .zip format!
Windows users can easily use Gnumeric Portable without even fully installing!
Compare CQMDX designs residential local noise performance with competitors designs in Lionel's table!
Flag Counter started on September 14th 2017...