Yagi Insulators - Suggestions ?

[VK7HH]

I like the pirtek clamp option as I no longer have a a bench drill and drilling accurate holes that align correctly in a boom with a hand held drill is problematic.

Can you substitute a Folded dipole DE with a Split Dipole DE in the same antenna design? The folded Dipole DE seems difficult at present with no pipe bender.

Hi again Brian,
Next time you travel to Bunnings if you haven't already got one, get a centre punch. Will make life much easier drilling by hand if that is all that is available.
http://www.bunnings.com.au/trojan-125mm ... h_p5612052

Also do not drill the roof screws in with the clamps with a drill. Use a socket set or similar.

Using a folded dipole will in most cases raise the feedpoint impedance to somewhere around 200 ohms. You will then need to build a 4:1 balun to match this back to 50 ohms.
Have you used MMANA antenna modelling software before? Put your yagi design dimensions in and have a play.

http://hamsoft.ca/pages/mmana-gal.php

Can be a bit difficult to get used to at first, but maybe you can load one of their test antenna files and have a look how it works.

[VK2XTC]

Bob,

I going to build two, the first with the old TV antenna parts and the second with all new parts and the Pertek clamps mentioned previously.

The TV antenna version I'm making light weight and portable, with the boom able to be cut in half to get it safely on the roof of the car. Maybe put it to use next weekend for the Summer Field Day.
The second one will be more robust and is destined for the roof of the house. I cannot get up on the roof and will have to pay someone to install it.
I only get to do it once so will put up something that will last and survive the odd cocky chewing on it. Plus I like building stuff !

The design for the TV antenna version is taken from http://www.k7mem.com/Electronic_Noteboo ... i_vhf.html with the settings as;

144.150 Mhz design Freq
2500 mm Boom Length (this is so I have some space to put a mount point behind the reflector if req on a 3000mm boom). also next size up is longer than 3m which Bunnings don't stock.
0.20 DL6WU Reflector Spacing
Metal Boom Insulated
Boom diameter 20mm
Boom correction - as calculated
Elements 10mm

7 elements Gain = 9.81dBi

However, I'm also looking at YU7EF design for 6 elements on a 2440mm boom. http://www.yu7ef.com/EF0206.htm
Interestingly it has less elements but 11.96dBi Gain. The folded dipole stumped me as the TV antenna elements I have are split rolled, which won't bend, hence the previous question.

I did find some lengths of copper pipe (5 Meters) under the house that was used for the old Gas Lights (very old house) , so may try that folded dipole afterall.




Brian

[VK2ZRH]

I, for one, will be interested in the outcome of your efforts.

[ZL1RS]

Hi Brian,

The short DL6WU Yagi's come with a warning on the output page:

Warning: The specified Boom Length of 2500 Millimeters is to Small. Boom Length should be greater than 2.2 Wavelengths.

To check the design is OK I put the calculated dimensions into 4NEC2. The result is quite OK except that the feed point impedance using a split dipole is around 40 ohms (thus 160 ohms with a folded dipole) and the feedline will carry a 1.25:1 SWR (not serious). A split dipole DE length of 989mm will be resonant in that Yagi design at around 144.2. BTW, the gain appears as 9.8dBd not dBi.

After a little manual fiddling, a better SWR can be obtained at the bottom end of the 2m band if the DE is slid forward to around the 470mm position on the boom ... the result in 4NEC2 shows more or less the same radiation pattern and near enough 50 ohms and resonant with a 998mm long split dipole DE (and thus 200 ohms with a correctly dimensioned folded dipole).

The YU7EF EF0206 Yagi has near enough the same gain, a slightly better radiation pattern (slightly smaller rear lobes) and a flatter SWR curve from 144 to 146 MHz. Pop also conveniently includes dimensions for a folded dipole to suit his design if the builder wants to use one.

Either Yagi will "work", and there would probably be little practical difference between them if they were subject to an A/B test.

73,

[VK2XTC]

Bob,

Very cool. Will have to find that software.

So based on K7MEM web site I had the following data.

Boom = 2500mm

Zero Reflector 1016.28 mm
415.95 mm D.E 999.29 mm
517.93 mm D1 915.00 mm
946.28 mm D2 905.77 mm
1393.42 mm D3 895.48 mm
1913.35 mm D4 885.57 mm
2495.68 mm D5 876.78 mm


My element cutting won't be that accurate so you can forget about the decimal point. If I'm within 1.5 mm I'd be happy.
The site states a 6.2mm dimensional tolerance for elements. Can you be that for out and still get it to work?

Are you suggesting to just move the D.E forward to the 470mm position (forward by 65mm ish ) and leave all other element positions the same , ??


(mission to Bunnings completed - I'm out of pocket $35 - time to start cutting )



Brian

[ZL1RS]

Check the element spacings Brian ... seems a touch of dyslexia in your listing with the D1 spacing

Edit: to answer the other questions ...

I rounded the dimensions to the nearest mm before putting them into 4NEC2 and used the uncorrected ... insulated boom ... dimensions for the software model. The lengths you have are the corrected lengths for "insulated mounting on a metal boom" which are the ones to use in practice for your intended build (assuming the insulation material is 'good' ... the PE material Pirtek and Stauff clamps are 'good', but some of the Nuxcom insulators and the required bolt through the element and boom require a different correction factor).

1.5mm cutting error will not be serious, but with care it is easy to cut them within 0.5mm ... or less for the pedantic builder armed with a file (looks in mirror ).

6.2mm dimensional tolerance ... I added and subtracted 6mm from the dimensions in the modelling software and the Yagi still looks OK, but why would you? ...

Yes, just move the DE forward 64mm (moving forward generally raises the impedance of the DE feed point, but also changes the resonance, hence the different DE lengths I mentioned in the previous post). After mounting the parasitic elements you might like to tape the DE in place temporarily so it can be moved around while checking with the SWR meter. Leave everything else the same ... except try the DE lengths I mentioned (or start long and trim back). As inferred 2 lines back, the 'DE position' and 'DE length for resonance' interact a bit, so be prepared to experiment ...

4NEC2 is a bit of a learning curve but is pretty good, and things will work out OK in practice as long as the correction factors for the particular element mounting method are applied. I understand YU7EF uses it to model his designs and they work when cut to the lengths given (again, as long as the element mounting correction is applied). He has hundreds of 'happy customers'.

73,

[VK2XTC]

Well it was a long day but the result is good.

A close up of the choke also showing a ferrite bead . I'll waterproof the connectors later.

and the finished antenna in the testing position.

The SWR is high in the centre of the band and low and each extreme, opposite to what I was hoping .
I'll play with the Driven element position tomorrow. And I decided to change it to a Metal Bonded with a bolt through the centre of the directors.
It was simpler to use what I already had from the Scraped TV antenna.

All up a good day.



Brian

[ZL1RS]

Hi Brian,

An observation from your photos ... the coax tails to the DE connection points are curved and with solder lugs, so are effectively adding say 5mm to each side of the driven element. Putting this overall 10mm of extra DE length into the antenna model the SWR curve is ...

with DE 10mm too long

... and sort of resembling your description of "SWR is high in the centre of the band and low and each extreme".

Before cutting the DE shorter, try moving the DE to the 470mm position as suggested and see if you get an SWR curve loosely resembling ...

with DE 10mm too long and moved to 470mm on the boom

Then slide a bolt into each DE tip leaving about 5mm of the head end extending to make the DE 10mm even longer and check that the SWR gets worse.

Then remove the bolts and cut a few mm off each end of the DE (say 3mm off each end to start with) and check the SWR again. According to the modelling, the DE at 470mm forward of the reflector and with a DE tip to tip length of 996mm (including coax tails and any curve there in, and solder lugs) should produce something similar to ...

with 996mm DE and moved to 470mm on the boom

The radiation pattern and F/B at the bottom end of the band will not substantially change as a result of fiddling with the DE position and length, but the SWR curve will change.

Don't expect the above SWR curves precisely because there are other things in the physical construction that make the real antenna different to the computer simulated one, but they should be in the ball park. And not sure about the effect of the metal trailer cage just under the antenna? ... or the antenna apparently beaming at the next house (with a metal gutter?) either?

Also, keep in mind that DL6WU based antenna designs are intended for the bottom end of the band, so don't expect a low SWR or a pretty radiation pattern at the 148MHz end of the band.

73,

[VK2XTC]

Bob,

I also woke in the middle of the night and realised that the choke is on the wrong side of the DE. I'll swap it over today.
Also cam up with a way to terminate the cable on an N Type Connector mounted to the boom.

I had never thought about putting a bolt into the end of the element as an adjustment point. Simple but Brilliant. and I suppose I can leave it there if I cut too short...
You will be pleased to know while building the Yagi yesterday I also knocked out a 15Litre bath of All Grain home brew beer using NZ Cascade Hops.
Ran the results through the program this morning and it seems to have hit all the numbers.

There was quite a bit of NZ influence on yesterdays activities..

I'm off to start adjustments.


and , I'll move the trailer.



Brian

[ZL1RS]

... a bit of that Cascade Hop beer and you won't care about the SWR

[VK2XTC]

Well today was a day of adjustments.

After moving the DE forward to the 470mm point , and a few cuts to the driven element it ended up less then 1.1:1 VSWR. The final D.E length was 981mm

I first cut away the Choke I made yesterday and started with fresh coax connection with tails as close to right angles as possible.
From there it was small cuts (3mm) at a time, watching the VSWR come down to close enough to 1:1.
At full power I can only just see movement on the SWR meter.

I then remade a new Choke on an old 35mm film canister that I found in the draw. It's 5 turns of RG-58A/U with a Ferrite bead next to the pig tails, terminates in a N type connector.
I live in Artarmon so I have Chokes on everything to keep to noise down.

See picture.

Tested across the band it looks like this.

144Mhz less than 1.1:1 VSWR
145Mhz 1.2:1 VSWR
146Mhz 1.35:1 VSWR
147Mhz 1.5:1 VSWR
148Mhz 1.4:1 VSWR

The final actual elements and spacing sizes are as follows;

Zero Reflector 1014mm (Error from computer value =2.28mm
470mm D.E 981mm 18mm (why so much.)
572mm D1 916mm 1mm
945mm D2 905mm 0mm
1390mm D3 895mm 0.48mm
1910mm D4 885mm 0.57mm
2493mm D5 877mm 0.22mm

Why the 18mm error for the driven element is interesting.

However, It's done . $35 for bits from Bunnings, a really blunt hacksaw and a Ryobi battery drill.

My "Pizza Coke and a Garlic bread" Yagi..

Brian

[ZL1RS]

Why the 18mm error for the driven element is interesting.

While an 18mm difference is more than I would have anticipated too, keep in mind that the physical Yagi built is not the same as the computer derived model ...

- several small dimensional changes (2mm at the REF, 1mm at D1 a 'sensitive' element, 1mm spacing difference at D2, 3mm spacing differences at D3, 4, 5)
- the elements are mounted on TV-style insulators of unknown properties (e.g. there is a significant effect of plastic loaded with carbon black uV stabiliser)
- the elements are fastened to the boom with a bolt (no longer "insulated from a metal boom")
- there appear to be rectangular aluminium reinforcements at the element centers (effectively a tapered element section)
- the Balun is an 'unknown quantity' (was its effectiveness as a Balun at 2m measured when cable tied against a metal boom?)

... but not a bad looking job for a 2m Yagi salvaged from an old TV aerial. How does it perform compared to what ever else you have?

73,

[VK7HH]

However, It's done . $35 for bits from Bunnings, a really blunt hacksaw and a Ryobi battery drill.

My "Pizza Coke and a Garlic bread" Yagi..

IMG_0056.JPG

Nice job Brian. What is the mounting method for the yagi? I see a U bolt drilled into the boom as if the mounting pole comes inline with the elements.

[VK2XTC]

Bob,

Performance wise it seems to do the job.
From the limited "Mobile Testing Platform " it seems to quite directional, Front to Back is Noise to S9. Nothing from from side. and generally raises a signal about 4 to 5 S-Points.

I'm more than happy for a first attempt.
I need to install the disconnect on the boom to enable it to fit on the roof racks of the car for Transport.

I'm now thinking about version 2. (and getting a new hacksaw blade )

Hayden,

I have used a U-bolt and drilled 2 positions about centre of the boom. One for Vertical and one for Horizontal use. Picture shows it in vertical position.
I needed to test it against a known source so a Repeater was first choice. I used VK2RDX Mt. Bingo in the Western Blue Mountains as the test. (Weakest signal repeater I could find)

My Vertical (Diamond X200A Dual Band) was just able to open the repeater with zero S meter and lots of noise, the Yagi brought it up to an S3 signal. (Distance of 110Kms)
When in the vertical position I used a 1.5m length of 38mm heavy walled electrical conduit (Orange) on the top of the quick extension pole.




Brian