Phased verticals for 17m (updated)

This week's work in progress is a return to several years ago, when I tried out a pair of phased, elevated verticals for 20m.  These worked well, but saw little service as my tower was due to be installed where they stood.

With more room now, I've ordered a couple of new 7m fishing poles and 75Ohm coax to make the necessary 1/4 wave stubs, this time for the 17m band.  You can either use the foregoing type of online calculator, or an antenna analyser.  In the latter case, you just connect an open-circuited piece of coax of slightly longer than the computed length to the analyser port and cut until the minimum impedance (close to or equal to zero Ohms) occurs at the operating frequency.

The basic feeding format for two phased verticals is, of course, pretty simple.  It goes like this, using 75 Ohm coax from the 'tee' to bring a proper match:

The devil, as always, is in the detail.  The maximum gain of two phased verticals, which is about 4.6dB over a single vertical (effectively taking 80W typial peak SSB up to over 220W), is achieved when the spacing between each antenna is at 5/8 wavelength, remaining at 4.5dB at 3/4 wavelength.  Gain falls away rapidly beyond 3/4 wave.

The pattern of phased verticals is markedly different from a single element, in that you achieve a bi-directional beam, 'broadside' to the line of the verticals, rather than along their line, roughly as shown in the polar plot below (imagine one antenna at 0 degrees, the other at 180 degrees):

Now, for 5/8 wave spacing, at 18.1MHz, the physical spacing needed between elements is about 10.4m.

But each stub has to take the velocity factor (in this case, 0.8) of the 75 Ohm coax into account, meaning each 1/4 wave leg is shorter: only 3.3m or so long.  Twice that is 6.6m.  This leaves us 10.4 minus 6.6 = 3.8m short (1.9m each side of the 'tee') for the correct spacing, and then only if the cable is dead-straight and doesn't have to reach an elevated feedpoint.  This is not the case for elevated verticals, and certainly not for vertical dipoles, yet is almost always ignored in books and articles!

Cutting my 18MHz stubs using SARK-110 analyser into an open circuit piece of coax.  Minimum impedance at ~18.1MHz indicates the correct length has been reached.  Note that the cable impedance selected within the analyser makes no difference to this analysis.

For coax that lies on the ground and has to climb, for this band, around 1.5m up a supporting fishing pole to the feedpoints either side, the total coax length is significantly longer than the diagram above implies if, as is likely to be the case, you are using a single, and not odd multiple of 1/4 wave for the stubs.

The shortfall is made up using the necessary length of 50 Ohm coax to reach the antenna feedpoints.

If we take that 1.5m as a reasonable feedpoint height, we need 3.3m of 75 Ohm coax as calculated, and then 1.9m minimum of 50 Ohm coax without any climbing to reach the bottom of each pole, and 1.5m on top of that to reach the feedpoint.  Total length comes out as 3.3 + 1.9  +1.5 = 6.7m per side.  


Overall, better to cut your stubs, then set up the antenna system in the field and see how much 50 Ohm coax is needed before cutting it off a drum!

And about 75 Ohm coax.  I bought some of this from a major UK ham outlet, simply because I didn't have any lying around and I wanted to see what they, as opposed to the usual TV/satellite cable purveyors sold.

Turns out the 'ultra low loss' ham-specific cable was the same thickness as RG-213, but had a very much stiffer, very plasticky coat that makes working and using it less easy than had I just bought the usual, more flexible TV type coax.  At 18MHz, it is of very little consequence as to whether the coax is 'ultra low loss' or not.  Even the technical specs admitted the difference between it and the cheaper,  'low loss' coax at 10MHz was only 0.6dB over 100m!

UPDATE:

Unlike some books, I like to both write about the theory, the building and the real-world testing of antennas.

So, if you are wondering if the above article actually yields a working antenna system, I have just spent four hours in the mud and rain setting it all up, and the answer is "yes!"

Firstly, the total coax length is just about OK, and does not necessitate any changes, but could preferably do with being about 0.5m longer (of the 50 Ohm coax) on each side of the 'tee'.

It took me a while to figure out why, initially, the SWR was well over 2:1.  This was not helped by a couple of hefty cows and their calves taking an interest in proceedings, but who decided to keep their distance eventually!  I tried moving the radials from along the beam to side on to it, but this made no difference.  There was some minor interaction with a nearby wire fence, and moving the radials away a little stopped that.

10m apart, giving a nice 4.6dB gain towards the camera (and the reverse, towards the trees) at 18MHz.

The first test day was very wet, with not only saturated ground but water running over the surface.  So the antennas could well have been appearing to be electrically longer than expected.  This is common to most wire antennas.  The excessive length could also easily be down to using PVC insulated wire.

Rather than cut the wire at this early stage, I just bent a guesstimated 5 inches (127mm) of wire on the vertical radiating elements and taped it up.  That length also turns out to be about 3% shorter - what we would expect with insulated wire, as noted above.  I left the radial length untouched, but did raise the feedpoint from 1m to to about 2m (using all sections but the top one of a 7m pole), and the ends of the radials to much the same height.

Returning to the analyser, I found this simple adjustment yielded the good match across the band I was looking for.  I could also see that moving the radials to slope markedly downwards tended to give a slightly poorer match.  This is not usually the case with single element verticals.

Below, you can see how the array's matching eventually worked out, after some equal pruning of the radials.  I also found the system has considerable sensitivity to the position of the feedline from the transceiver itself to the coax 'tee'.  Like all antennas, it's important to bring the feedline away in as symmetrical a manner as possible.  The match quickly becomes poor if the coax runs so close to one of the antennas that it lies within the diameter of the radial field.

Unfortunately, and in addition to lots of QSB, 3B7A was active with an enormous split pile-up taking a very greedy 15kHz of a very narrow 17m band.  But I did manage to work 4X4FR at 59+ to me from his dipole, 55 from my array, probably completely out of alignment with one another's 'beams'.  I later managed UR4MSF at a good 59 each way.  That's actually not bad going, and in accordance with the propagation model, given the SFI of 67 and consequently quite terrible band conditions at solar minimum on this band. 

My SSB QSO's in agreement with the ITUPROP model.

I also heard a JA station calling CQ, but could not change the verticals around for the correct beam heading quickly enough.  Nobody was coming back to him, so he vanished.

The gain/null of the phased verticals is very noticeable.  I could hear a Florida station in the US at about 54 rise to about 56-7 as I moved one vertical from a N-S pattern to give a E-W beam pattern.  I could also hear the audio rise very clearly from outside the shack.  A US-UK path was not predicted in the model.

The following day, I worked VO1CAL and KF4ZZY at 57 and 54, respectively, when the band conditions were very terrible and everyone was using 500W-1kW.  I was only using about 80W peak output.

The array is not very cumbersome to set up or set down, and simple clothes line ground screws are more than enough to fix the fishing poles in place (in the UK, Wilko currently sell good, wide ground screws for £6 each).  It's something that can easily be carried in the car.  It's certainly a good system, with a good degree of gain for very little outlay.  If you use kevlar-coated wire rather than equipment wire, this makes for infinitely fewer wire tangles, and is much to be recommended for avoiding angry outbursts on setting up!