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I am in the process of planning a multiband vertical antenna and have a few questions I would be grateful for any advice on.

So I plan to have 1/4 wave elements for bands 20-10m with a common feedpoint and shared radials, something akin to the commercially available DX Commander antenna, whereby elements self select on the basis of desired impedance. I would like to include 40m also , but there is one question I have regarding this:-

So I have found a supplier here in the UK that can supply a 6m long fibreglass pole/tube. I think this will be a good solution as unlike the telescopic poles sold by many ham radio outlets, this will be the same diameter throughout the whole length (around 45mm with a wall thickness of 2.5mm). My plan is to have around 50cm of the pole below ground, sitting in a sleeve made from a length of PVC pipe concreted in place. So with this in mind, the pole will be long enough to support elements for each band from 20 to 10m using plastic plates as guides for the elements (basically a copy of the DX Commander antenna). My idea for 40m is to make the element linear loaded so the wire extends up the pole, back down and then back up in such a way that it is the desired length. The question I have is will this linear loaded element present near enough a 50R impedance, I will tune all the elements using an antenna analyser, but I just want to know if a 50R or close enough will be easy to achieve? My aim is for the whole thing of each band to be tunable with my rig's (ft991a) internal tuner. The other option I am considering for 40 is to make the element an inverted L, which would be ok but not as neat mechanically as the linear loaded version. I realise that an inverted L might enable some local contacts due to the horizontal section, but I primarily am interested in making DX contacts with the antenna, I have an OCFD which certainly for 40m works well for NVIS contacts due to its limited height. Can anyone tell me which of the two variants (inverted L or linear loaded vertical) is likely to yield the best results for DX? Or if the difference is unlikely to be noticeable.

Many thanks for your help.

73

Owen 2E0IHR

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    $\begingroup$ Note that a ground mounted antenna with presumably a ground plane 90 degrees to the radiator, will have a feed-point impedance closer to 37 Ohms, not 50 Ohms. You can attempt to raise the feed-point impedance by making the radiating elements longer and the ground plane elements commensurately shorter, although being ground-mounted this technique still may not raise your feed-point impedance much; the other option is a 1.5:1 UnUn. And keep in mind that the "self-selecting" feature of any Fan antenna, is purely a matter of a signal traveling on the wire with the lowest feed-point impedance. $\endgroup$ Commented Oct 31, 2023 at 11:17

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An inverted L is a far, far superior solution to any sort of loaded monopole. The antenna is effectively full size, you get a whole quarter-wave of wire in the air, low and stable SWR across a reasonable band. If you're already stringing up guy ropes and things, I recommend the inverted L. If it can't go exactly sideways, go as far as you can and adjust the length until it resonates.


Some notes about linear loading for 40 m:

Linear loading isn't so much about the total length of the wire used; it's a way of getting some inductive (or capacitive) reactance without using discrete components. It works by creating a short, short-circuited stub, in series with the antenna. Like this:

schematic

simulate this circuit – Schematic created using CircuitLab

The impedance of the stub can be calculated by:

$X_{stub}=jZ_0\tan(\beta x)$

where $\beta={2\pi\over,{\lambda}}$

DX Commander doesn't seem to use this folded-back method of loading, and their poles are so long they're full size on 40 m. But some quick estimates for dimensions for your case:

This much reactance can be achieved, barely, with the following stub (calculation):

  • $Z_0$ of the stub: 300 ohms. So the spacing of the two wires should be about 10 x their diameter.
  • Length = 5.0 metres. So fold back at the top and come back down almost to the feed.

Note that at this length, the whole arrangement is extremely sensitive to the exact spacing of the wires (as it blows in the wind...), and the exact length. For example, if the spacing changes by 10% then the impedance will change by 10%, giving you an extra j50 ohms, immediately raising your SWR over 2:1.

Also if you change frequency, say by 1% (7030 to 7100 kHz) then the antenna reactance changes by only +j5 ohms, but the stub reactance changes by +55 ohms, giving you an SWR over 2:1 again. This is the curse of short antennas in general; but the linear loading stub is much worse behaved than a regular inductor, which would have changed inductance by only about 5 ohms)


Second, about feedpoint resistance:

From antenna-theory again, the radiation resistance will be about 3 ohms, plus ground losses, which depend strongly on your ground system. If you find the SWR at resonance is under 5:1 (10 ohms) you can probably manage this with your ATU. Better would be to install a parallel capacitor at the feed. Ground losses also "help" with the bandwidth problem mentioned above.

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