Off-Centre Fed Dipole Question/s

[VK2CSW]

I had a thought about why the OCF legs are appearing to be far too long.

The original overall length (OAL) was to be 20.8m and was cut at a bit over 21m for headroom. Currently the OAL is less than 20m and it will be closer to 18m if I fold or cut it according to the M0MCX calculation. Which seems far too short for the OAL for a 40m dipole.

Below is a rough sketch of the layout. Forgive the diagram, you wouldn't think I am an illustrator by trade, but I don't have any real tools here...

   

It has a 4:1 balun at the dipole's division, coax down to the cable entry where there is a choke (coax on a toroid) some more coax to another choke at where it would connect to the tuner, but in this case it connects to the NanoVNA.

In this arrangement I suspect it is actually behaving like a Windom style antenna rather than a dipole. I think the coax between balun and entry choke is forming part of the antenna.

The next test will be to move the entry choke to next to the balun and do a sweep and see where the lowest VSWR/Resonance ends up. Happily, to do that doesn't require a ladder, the design using pulleys and halyards allows me to lower everything to ground level)

If the lowest VSWR point moves higher in frequency then, as Adam Savage was wont to say "Well there's your problem...".

(Forgive me if my thinking out loud here is annoying).

[VK4ADC]

Colin

The proximity of the wire components to the large metal shed roofs will also contrbute extra capacitance (effectively to ground) where in a truly free-up-in-the-air mounting, that capacitance would be far lower.  The effect : lower resonant frequency.
Recall F = 1/2*Pi* square root LC ; where increasing C reduces F

To solve, you either need to raise the height of the OCF substantially (reducing C) or orient it sideways over unobstructed ground (ie no sheds) {reducing C) or, as you have done, reduce the L component - due to length - to shift the frequency upwards.
Unobstructed does not necessarily mean no trees, but a greater distance above ground.(reducing C)

Gotta be reasons why we have to learn electronics / radio theory to qualify for amateur radio licences !!  Just a matter of applying the theory.

Doug

[VK5PJ]

Hi Colin,
what bands are you wanting to cover with the OCF dipole?  I have been using one here for near on 20 years (primary HF antenna).  Many moons ago there was a Ham in VK3 that advertised them and as a new bloke to HF (I was a Z call) it seemed like a easy spot to start. He offered a few different offset ratio's in his OCF dipoles.  As hard as I try I cannot remember the dimensions of mine but I did rebuild it with a new 4:1 BALUN and then new wire (Grandfathers Axe I guess) but it has always been a good performer for me.  Usable on 40,30,20,17,15,12 & 10 mx only using the internal ATU of the rig.  It is near as long as a 80M dipole but of course does not tune on 80m without treating it as a "long wire" type of tune.  I wish I had kept the original doco for it.  Might have to connect it to the ant analyser to see what proper dips it has but have resisted fiddling as then it might break the magic spell.

now to be fair I must declare it hangs from the top of a southern cross tower but is in an inverted V form (anchored into trees).

I will keep reading and I might learn something along the way.

[VK2CSW]

Hi Doug,

In line with your thoughts I also have a 20m dipole parallel to the OCF at 1m away. I dropped this down and reswept - no change.

I then moved the location of the entry choke to directly under the balun. Again no changes.

So next will be to fold back each end of the OCF as described above. Ran out of light and will power tonight.

Hi Peter,

It will be nominally 40, 20 and 10.

As an experiment later, possibly a thought experiment at first, given how far I need to shorten it due to the coupling, I will look at trying to double the shortened lengths to make it an 80m one albeit shortened. It might be a struggle...

[VK2CSW]

Okie Dokie,

Got out early to beat the wind and forecast storms

I trimmed in accordance with the M0MCX calculator.

6 to 7.5MHz sweep is now:

   

The wider 6 to 29.5 MHz sweep is:
   

Small adjustments at either or both ends starts to push the VSWR up again. So I am calling it as good as it will get. I run a tuner in any case and it is within that range 

Next tests will be on air.

Next experiment will be to pull it down and measure the length of each leg, then work out if I can fit it in if I double it...

I shall report back with any new results!

I am concerned that fiddling with bits of wire is strangely appealing.

Edit: The final measurements for the 40m OCF were 6.3 and 12.2 m. OAL 18.5 m.

So 37ish metres for an 80m OCF. With some significant tree pruning and minor engineering it might come close.

[VK2CSW]

Tuned it up on 20m. Answered a CQ Parks call from west of Gosford NSW.

I guess it works.

Time will tell how well it works.

[VK2CSW]

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.

Finally got to Roseworthy Elders.

Picked up 10, so I guess there is more wire wrangling to come...

[VK2CSW]

Finally got to raise the centre support a bit over a metre.

It is now tending towards being on OCF inverted V.

VSWR at 40m came down slightly and pushed slightly higher up the band - as expected. I will shorten it a bit but it won't have much effect.

Still pondering whether I can stretch to doubling the size...

[VK2CSW]

After playing for a bit, I have found that I can tune the OCF on 40, 30, 20, 17 and 10m.

Can't quite stretch it to 15 and 12m. The tuner doesn't run to 6m (although I have a cheap Chinese tuner on the way that is supposed to cover to 54Mhz).

So of course this has tweaked my curiosity about making a fan dipole for 15 and 12m. I have knocked up a choke/balun with two sets of antenna terminations. I am thinking about using some 13mm or 25mm poly pipe as spreaders.

I am still considering the four "end" spreaders as they will need to have more rigidity than the others spreaders as I don't think poly pipe will take the compressive load.

Having said that, it will cost very little to try with poly pipe I guess! The vineyards give away kilometres of old pipe when they replace the irrigation on a block.

[VK3RX]

I've made a couple of fan dipoles over the years, spreaders being that black garden irrigation tubing that is about 15mm diameter. I also use the same for the feeder for my HF doublet. See pix.

The fan dipole end pieces were sprinkler fitting tubes i.e. about 8mm diameter hard plastic that would have a nozzle  fitting on the end and the other end punches into the water feeder tube. Or you could use this type as spreaders too, if weight is an issue.

Fan dipoles are a mongrel to tune though - much raising and lowering and fiddling overall lengths of each dipole, which obviously interact with each other. A two band type would be easier to tune though.

       

[VK2CSW]

Hey Damien,

You have just triggered a thought and a mental redesign, as I am almost positive that in one of the boxes of irrigation odds and sods we inherited when we bought this place, there are some 19 or 25mm riser pipes - the rigid ones you can mount a tap on to.

I might go and have a discussion with the snakes and spiders that call that part of the back shed home and see if I can find a couple...

Cheers!

[VK3RX]

Might be a bit heavy?

The garden sprinkler stuff is light and stays in place, if you drill the holes to be a tight fit for the wires - although with the spacers I added a touch of Super Glue just in case ....

[VK2CSW]

Good point.

Still, a visit to the dark and dingy corner of the shed is in the plan. Who knows what might turns up? (Other than Brown snakes and Red-back spiders, that is...)