Off-Centre Fed Dipole Question/s

[VK2CSW]

For some reason I started looking for alternate antenna ideas, again.

I came across on article on the FAMPARC website about an 80m OCF which also described a 40m OCF.

It piqued my interest, so naturally, down the rabbit hole I went.

One question I was unable to work out an answer for is does it matter which side of the antenna is connected to coax centre core?

Also do any of my VK5 neighbours have a local source for lightweight plastic insulators?

I have previously bought them at Ag suppliers (i.e. electric fence insulators) but I have only been able to find the ceramic ones or the stand-off types.

[VK5TM]

Hammer Barn has a version meant for wooden posts, not sure if they are usable for antenna construction (ceramic ones are cheaper).

The egg shaped plastic ones seem to be a rarity at the moment.

[VK2CSW]

Thanks Terry.

I haven't tried the ag places a bit further out - might be an excuse for a run towards the river...

[VK5TM]

If you're friendly with any of the local farmers or horse owners, maybe ask them if they have an odd one or two lying around.

[VK2CSW]

Not a bad idea, thanks Terry.

Random text to make the character limit.
You know you're getting old when the radio stations and bars play music you don't like, but the supermarket is throwing out banger after banger.

[VK2CSW]

I ducked into Hammerbarn on my way home today.

Their website said they had some white GRP strain insulators in stock.

Of course, they didn't. While wandering the aisles looking for ideas (and finding a 100mm bench vice for $39) I walked past the irrigation fittings.

It dawned on me that some 13mm polypipe, pipe shears, a 4mm drill bit and a drill press would see some handy dandy custom insulators made up lickity split. All of those things are already in the shed.

I also remembered to finally get some small gal eyebolts for strain posts on the balun and choke for the antenna.

Result!

Insulators? Check.
Halyard and support rope? Check.
Eyebolts? Check.
Toroids? Check.
Enamel wire? Check.
SO235s? Check.
Junction Boxes? Check.
Wire? According to Auspost it's on it's way.
Time? Bugger - house full of visitors to entertain for the next week or so...

[VK2CSW]

Running out of excuses...




   

[VK5TM]

Had to go in to my local Ag store yesterday and asked about the plastic egg style insulators.

They are still available, but only to order as they are slow sellers - the ceramic ones are preferred by the farmers down this way.

[VK2CSW]

Cheers Terry.

I might go and rattle Farmer John or Nutrien's cages.

Or I might put some polypipe offcuts to work...

But not today! Too miserable outside and we have a long lunch planned at Kaleske Wines instead.

[VK2CSW]

I managed to get hold of some plastic strain insulators at Farmer Johns at Nuriootpa. A bit larger than I wanted, but they seem to be hen's teeth.

I got to work on the 4:1 balun and it turned out quite well. Two stacked FT140-43 toroids inside some 50mm PVC pressure pipe and end caps.

Put some resistors adding up to about 250 ohms across the terminals and swept it with a NanoVNA. At 7.0 to 7.3 MHz it transformed the resistance to just under 50 ohms.

Good enough for me!

[VK2CSW]

The OCF dipole is pretty much ready to go up. I even have a couple of hours spare tomorrow.

Of course this is the forecast...


   

[VK4ADC]

Colin

The questionable weather seems continually attached to AR events so get the OCF up in one of the pauses in the rain.. Even if you don't dot all the I's and cross all the T's - that can come later if you like the results.

When I was a regular field day contester, virtually every event was accompanied by rain .. and .. thunderstorms .. and I wouldn't even like to guess how many times I packed up the portable station in the rain. Everything ended up wet including me.  Followed by a not-so-pleasant drive home in wet clothes.  It got to the point that I added an extra set of clothes to my FD packing list so I would at least be dry for the trip home.

Don't forget to post some images of the nanoVNC scans.

Keep up the fine progress reporting (or should that be wet reporting this time ?)

Doug

[VK2CSW]

I am not concerned in the least by the rain, as a wise Warrant Officer once screamed in my direction "the Good Lord made sure all the important holes point downwards, so you will not drown in the rain!".

Thunderstorms on the other hand...  We have only be here for three years (in the Barossa) and I haven't yet come to terms with storm watching - they seem to come from nowhere here.

On the east coast you can see and feel them coming. Not so much here, well at least for me.

When the current shower or two passes I hope to get the antenna aloft, with a weather eye working overtime!

[VK5TM]

I have to wonder sometimes about peoples comments and not getting wet - do they take an umbrella in the shower?

Normally I use https://www.blitzortung.org/en/live_ligh...php?map=20 to keep an eye out if thunderstorms are (supposedly) forecast, but it seems to be somewhat hit and miss lately and yes, they occasionally appear from nowhere down here too.

[VK2XAX]

I'm surprised that no one has mentioned Bunnings as yet, they have electric fence insulators thus...

https://www.bunnings.com.au/o-briens-whi...k_p3040955

[VK2CSW]

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In amongst in in the showers I got the antenna aloft.

This first image is the sweep from 7 to 29MHz on the uncut antenna.

   

The next is the point where any change at either end caused the VSWR to start to go back up. This also coincided with a break in play for lunch.
"Gentlemen, that's over and lunch".

   

As I was packing up the NanoVNA I realised that I am going about this all wrong. I did a quick sweep of 6.5 to 7.3 MHz and found that the antenna is still way too long.

   

Lowest VSWR is around 6.5 MHz.

When play next recommences I will try a radical fold back.

M0MCX has a calculator that suggests that if this were a dipole it would be almost 1.8 metres too long. 

https://www.m0mcx.co.uk/quick-swr-calcul...-antennas/

So I will adjust the length accordingly (after dividing by 1/3 for the OCF split ((0.9*2)/3) gives me a 1.2m and 0.6m adjustment.

To do this one of the halyards will need to be lengthened, so this is for future me to do.

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I'm surprised that no one has mentioned Bunnings as yet, they have electric fence insulators thus...

https://www.bunnings.com.au/o-briens-whi...k_p3040955

Bunnings say they carry them and have them in stock.

Even after ringing the store to assure they are in stock, upon arrival nothing, zip, nada!

[VK2XAX]

Well that's annoying!, they have other fence stuff too that may help. Sometimes things get pushed to the back or left on the top shelf , or so I've found at my local outlet.

[VK2CSW]

I actually found the empty box, pushed to the back of the shelf...

BGSFOS* aren't what they used to be!

*Big Green Shed Full Of Stuff"

[VK5PJ]

Colin (VK5SCW),
you need to go to Roseworthy, there is an Elders Rural there and they have BAGS of the plastic insulators, I bought 3 today, about $2 a pop...

As you go in the show room, turn left and its in the opposite corner, next to the Waratah Fencing display.

[VK2CSW]

Cheers Peter.

I will drop past on my way home next week.