Horizontal support for suspending half wave dipole elements

[M1BWT]

I'm currently using a half wave inverted V fan dipole, with a centre support and no possibility of raising the ends. I'm wondering if the smallest 10m element could be suspended horizontally from say a ~5m piece of light but rigid plastic tubing, such that it forms a T at the top of the mast (see below picture).

dipole_t.png

I was thinking of having copper stranded wire of the appropriate length supported at each end of the tubing, and that would allow the horizontal tube to be attached to the top of the mast. The element wire could sag a little underneath for a bit of extra clearance (in red in the above picture).

Has anybody had an experience of stringing a dipole element in this way – I'd be interested to hear comments as to how the close proximity of the tubing might affect the dipole element, perhaps you'd see skin affect on tubing, or it'd otherwise impact the radiation pattern &/or input impedance significantly? Perhaps a wooden pole (if it was structurally possible) would be better? Maybe you could even rotate it for directivity easily then too!

[K7KBN]

I'm a bit confused, Simon. You start off with talking about an inverted vee and then segue into what sounds like a yagi ("...smallest 10m element..."). If I'm reading between the lines correctly, you want to make a dipole for 10 meters, but conceal (and support) the wire more horizontally by using something like PVC tubing.

Should be workable, but keep a bit of tension on the wire so it doesn't coil up or pull back toward the center.

Or, why not make a dipole out of maybe 1/2 inch aluminum (okay, aluminium) tubing? Split at the center (okay, centre), attached to a piece of well-sealed wood or other reasonable insulator. For 10M, that shouldn't be too heavy or cumbersome. It may be directional but I wouldn't think it would be drastically so. If it is, you can rotate it with about anything.

[M1BWT]

Hi Pat,

Sorry – to clarify it’s presently configured as an inverted V with elements for 10, 15, 17 & 20m. I was thinking of supporting horizontally just the 10m half wave dipole element via the T configuration in the diagram, as I’d most likely be able to support that mechanically (there’s no other way to raise the legs up in this situation). But reading between the lines yes I’m interested in supporting (not worried about concealing) a single half wave dipole for 10m.

Making it out of aluminium tubing would perhaps be the next step – initially was just after an easy way to get the dipole horizontal with a single support.

Thanks for the tips.

[K7KBN]

You might want to give some consideration to making an aluminum dipole for 20M and feeding it with parallel feeders (twin lead; window line; etc) through a matching network. Should be able to cover all five bands efficiently.

[M1BWT]

Unfortunately don’t have quite enough space for that (the ends would be over hanging neighbours roofs) – but a potential option for just 10M.

I think I’ll give a bit of PVC or perhaps some thin fibreglass pipe a go at suspending the ends of the dipole for 10M, and look at using aluminium for the actual element as a next step.

Cheers.

[M1BWT]

I put up a 10m dipole suspended horizontally by PVC tubing (so that the dipole wire was not in contact) up at about 16m from ground - supported 6m from roof – see below picture.

ant.JPG

I was using the same dipole wire, balun, coax and mast as in a previous setup where the dipole was configured as an inverted V, so only changes were:

1. addition of the PVC support pipe
2. moving the balun ~ 500mm down the top aluminium support pole to the T formed by the PVC pipe
3. dipole changed to flat-top rather than inverted-V

Interestingly, even after various adjustments of element length, the minimum SWR would not drop below ~ 1.7:1 – adjusting the length just moved the frequency where this was obtained in the band. However, previously in an inverted-V configuration it’s always been straight forward to reach resonance.

What I’d like to understand better is why it’s bottoming out at 1.7:1 as a flat-top (more an understanding thing than quest for 1.1:1!) – my first thoughts we some unwanted interactions?

• Interactions between the dipole and aluminium support pole, possibly made worse by the fact that the balun has been moved down the aluminium pole, rather than being right at the top. Didn’t appear to cause an issue when in inverted-V, however.
• Some interactions between the dipole and the PVC support pipe, although I can’t quite see how.
• The dipole does have an area of roof within 0.5 wave lengths at 10m, which was the same when it was an inverted-V, but perhaps the expanded wave pattern of being a flat-top now is causing more interactions. That said I’ve not suffered with a 20m inverted-V in the same position.

Suggestions anyone?

[K7KBN]

Try adding maybe 2-3M of feedline (use a double female adapter) and see if that changes anything. You really need to know the feedpoint impedance and not what the transmitter is seeing some distance from the feedpoint.

A 10M dipole sitting 16M above the ground might be a better candidate for 75 ohm feedline (I assume you're using 50 ohm stuff now?)

[M1BWT]

Yes it’s just 50 ohm coax for feeder at present, a run of 16m, which is 6.4 ¼ wave lengths @ 10m so I’d thought that length would be okay. But good tip though, I’ll give adding a bit more feedline a go to see if that makes any difference.

[M1BWT]

The penny dropped after a bit more research & re-reading Pat’s post – thought I’d close off the thread with a quick explanation, which will hopefully help anybody else with the same query.

The impedance of a dipole antenna (e.g. 73 + j45 Ω) consists of resistive (73 Ω) and reactive (j45 Ω) elements – with the reactive element either being capacitive (expressed as a negative number) or inductive (positive number). The antenna is in resonance when the reactive element crosses zero, i.e. the impedance is purely resistive with no reactive element.

Adjusting the dipole leg lengths for resonance primarily alters the reactive element of the impedance, so at the point of half-wave resonance even though the reactive element is near zero, the resistive element might not change significantly.

With half-wave dipoles, the impedance of a high (in terms of wavelength) flat-top half-wave dipole approaches 73 + j45Ω, whereas an inverted-V (with 90 degrees between the legs) at a similar height has a resistive impedance nearer 45Ω.

So, feeding the inverted-V with 50Ω coax presents a good match, whereas the same antenna but as a flat-top gives a mismatch between the 50Ω coax and lets say ~75Ω antenna feed-point.

My mistake here was that I believed adjusting the dipole legs would primarily alter the resistive impedance element, but in fact that’s more determined by other factors (height and dipole setup in this case).

So with 50Ω coax into a high flat-top half-wave dipole you’d expect at best ~ 1.5 VSWR without any other matching in place.