VHF/UHF Long Yagi Workshop

A good Yagi design is as easy to build as a poor one...

SO,  ALWAYS  BUILD  A  GOOD  DESIGN !

For 144MHz long yagis, the standard reference for comparing many different designs is by VE7BQH: A G/T Study of Two Meter Yagi Systems.

Articles from DUBUS

• DJ9BV: Yagis for 6m (1998)
• DJ9BV: BVO2 Yagis (1998)
• DJ9BV: OPT70 Yagis (1998)

Full details of these three ranges of Yagis are in
DUBUS Technik V  (Warning: Errors in printed data)

• DJ9BV: Yagi Antennas for 144MHz (1990)
• DJ9BV: High Gain Yagis for 432MHz (1991)
• DJ9BV: DL6WU Yagis for 23cm
• DL6WU Yagi for 2320MHz

Programs to design "classic" DL6WU Yagis

As close as possible to DL6WU's original designs and intentions:

• Extremely Long Yagi Antennas, VHF Communications, 3/82
• Zelf Ontwerpen en Bouwen van VHF en UHF Antennes (with PA0MS), VERON
• Supplemented by direct information from DL6WU.

• Includes element mounting corrections
• Clear printouts of dimensions
• Updated using beamwidth estimates from NEC calculations by VK1OD.
• Stacking distances now use only the "classic" DL6WU formula.

Downloads

• Qbasic source code (text file)
• Standalone .EXE file (runs in MS-DOS window)
• Excel/VBA spreadsheet by VK1OD (requires Excel)
• Online DL6WU Yagi Designer by K7MEM (runs in your web browser).

Other Yagi design links

W4RNL's articles on "OWA" (Optimized Wideband Antenna) Yagis

YU7EF
"Low-temperature" antennas with clean patterns, all optimized for 50 ohm feed.

DK7ZB
More good designs, cleverly optimized for easy matching.

G4CQM
Wide range of 50-ohm yagi designs featuring the direct coaxial dipole feed (including kits and insulators for construction).

G0KSC
Lots of good modern Yagi designs, including OWA yagis, Justin's unique Loop Fed Array Yagis, and his new low-impedance OWL designs  (also a range of insulators for construction).

W1JR's articles on Yagi designs  for Astron Antennas

Stacking Yagis for the same or different bands

Computer Modeling

• DL6WU and DJ9BV: Yagi Simulation ( 1991-92)
  Part 1, Part 2, Part 3, and Part 4.

• VE7BQH: Simulations of 432MHz, 144MHz and 50MHz Yagi Systems
  Charts of G/T and Gain versus boom length.

• W7EL - EZNEC modeling software

Element and Boom Corrections

 CONTENTS

End, Back

IMPORTANT WARNING!

If you are copying an existing Yagi design, you must copy:

• all the element lengths
• all the element diameters
• the element mounting method

Element diameter corrections

The resonance of each individual element depends on two key dimensions:

1. Element length
2. Element diameter

For VHF/UHF Yagis we can assume the elements are made from a single piece of rod or tube with constant diameter. (Many HF Yagi elements which are made from telescoping tubes of different diameters, which adds further complications.)

If you propose to change the element diameter, you will need to change all the element lengths!

For small corrections, it's safe to make the correction using either of two methods:

• Make the resonant frequency of the new element the same as the resonant frequency of the old one
• Make the reactance of the new element at the operating frequency the same as the reactance of the old element.

Obviously these two methods are very closely related, so they are equally valid.

Boom-effect corrections

The resonant frequency of every element is affected by any metal near the centre of the element (where the magnetic field is strongest). That means that the length of each element needs to be corrected for the presence of a metal boom - especially if the element is attached to the outside of the boom or passes through it.

• Yagi Element Mounting - by W1JR
• Boom Corrections - by SM5BSZ
• Boom Corrections - by VK2KU (Click the Antennas and Feeders button, then click QEX Article)
• Scaling and Adjusting VHF/UHF Yagis - by W4RNL

I suggest you read all four of these articles, because each one has a slightly different viewpoint.

Also, each of the first three articles suggests slightly different boom corrections - and the program below uses yet another method which is different again. Fortunately the boom correction is usually small, so it does not have to be super-accurate. If you are worried about the accuracy of boom corrections, then use an element mounting method that only requires a small correction!

If you are still worried, there is no substitute for making measurements of the boom correction for your own chosen mounting method - the articles give details.

A useful formula for boom corrections is:

BC = [733 * BD * (.055 - BD)] - [504 * BD * (.03 - BD)]

where BC is the correction as a fraction of boom diameter BD (both in units of wavelengths). Boom-effect correction must always be added to the uncorrected element length.

This formula is derived from experimental work by DL6WU, and applies to elements passing through a round boom with electrical connection on both sides. Most other mounting methods require a smaller correction, eg the correction for insulated elements passing through the boom is usually taken to be about 0.5 of the above.

Some practical perspective: don't agonize too much about the accuracy of boom corrections. The important parameter is the electrical length of the element, so any error in a correction is only a second-order effect. An accurate correction will only be needed if the boom effect is large... and in that case, the best solution is to change to a different mounting method that doesn't require such a large correction.

Here is a program that might help you to convert from an existing ("source") yagi design to a new ("destination") type of construction and element mounting. It works like this:

1. Enter details of source yagi construction.

2. Enter details of destination yagi construction.

3. For each element in turn:

1. Remove any boom corrections from source yagi dimensions

2. Calculate a new element length for the destination yagi construction (using the reactance method outlined above)

3. Apply any new boom correction for the destination yagi

(NB: this program does not calculate boom corrections for steps 1 and 3 - you have to do that separately.)

The program uses simplified methods, but it will be accurate enough for reasonably small changes in construction - for example, to convert a European design from standard millimetre tubing sizes into the nearest available inch sizes. But always remember: the more drastic the changes you want to make between the source and destination methods of construction, the greater is the risk of conversion errors becoming significant.

CONTENTS

Practical Construction

  Aerial-Parts of Colchester (G4ZTR) for specialised antenna and tower parts including element clamps, boom/mast clamps, boom couplers, power dividers and other antenna hardware.

Articles

Folded Dipoles for VHF/UHF Yagis

Ideas for element mounting

I strongly recommend using insulated elements, mounted through the boom. They are easy to make, and the long-term performance is more constant than almost any mounting method that requires electrical contact between the element and the boom.

Nylon rivets for 4mm elements, insulated through the boom

This is the 'classic' European method, much used by DJ9BV and followers with 4mm rod elements. US designers tend to use 3/16in (4.76mm) elements, with plastic shoulder washers and push-on metal 'keepers'.

These nylon rivets are actually intended for fastening two sheets of metal together (push the rivet through both sheets, and then drive in the pre-formed peg to expand and lock the fingers). But hams discovered that if you snap off the peg instead, leaving a clear hole through the middle, the nylon bush is a firm drive fit for 4mm rod.

You need two rivets per element, inserted from either side of the boom as shown here -

• When drilling the boom, find the drill diameter that keeps the rivets firmly held, but without squeezing them.
• Cut a 1mm 45deg taper on the ends of the elements (pencil sharpener held at an angle) to help them pass through the rivets.
• Keep the rivets soft in a bucket of hot water until ready for use.
• Insert one pair of rivets at a time, and then drive that element in.
• Support the lower rivet and the boom in a Workmate bench vice, or else the rivet will pop out when you drive the element through.
• Make a driving tool from a length of tubing which is a slide fit over the element, held in a wood or plastic chisel/file handle.

Other plastic insulators

Plastic shoulder washers are available from various sources... just keep your eyes open!

In the USA, you can get the black Delrin shoulder washers and keepers from Directive Systems.

W7XC has had good results with ordinary plastic wall plugs that have flanged end. Find the size that is a push fit over the element rod, and cut off the excess length inside the boom.

Heat-shrink tubing, through the boom

SM2CEW described this simple, low-cost element mounting method in DUBUS a few years ago (reprinted in DUBUS Technik V).

Peter uses ordinary heat-shrink sleeving for the centre insulation, and he fixes the elements through the boom using a hot-melt glue gun. He lives close to the Arctic circle, and after several years the elements and their mountings are still holding up well against the extreme weather. 

"Each element is insulated from the boom with a 35mm long piece of heat-shrink tubing [for boom diameters up to 32mm].  A 4.9mm hole is drilled through the boom [for 4mm diameter elements] and the element with its heat-shrink tubing in the centre is put in place. It is then fixed in place by gluing it to the boom with melt glue from an electric glue-gun. Enough glue must be used to flow around the whole circumference of the element for the best results. The result is a very neat looking yagi with minimum size holes through the boom."

This idea is hard to beat for low cost and simplicity!

 CONTENTS

1. Design Information
2. Computer Modeling
3. Element and Boom Corrections
4. Practical Construction