Martin, I can comment on your plan to use relays to short out the
inactive drivers. It adds unnecessary complications. I do agree that
it is essential to short the inactive driver elements, but that can be
done by using electrical quarter wavelengths of coax between the boom
mounted switch outputs and the the driver feedpoints. If you leave the
switch end of the coax links open when they're inactive, the feed end
will be shorted for RF of that frequency.
I didn't follow this requirement on the first four element quad that I
built many years ago and it worked poorly and didn't conform to the
antenna models. After getting advice from LB regarding the quarter
wavelength links, I have successfully built a couple of four element six
band quads and they have worked well and behaved pretty much as
predicted by EZNEC.
In fact, I measured the SWR and impedance characteristics of one of
those quads with the inactive bands opened vs shorted, and there were
substantial differences between the two conditions on most bands. (The
antenna was very close to the ground for these measurements and mounted
on a 12 foot ladder.)
All that being said, you have a two element quad and the feed impedances
should be in the vicinity of 100-140 ohms. Therefore, you'll need
quarter wave links of 72 ohm coax between the switch and the feedpoints
to match 50 ohms. They should be left open at the switch end as
described above.
You can use the eight foot spacing for 10 and 12 meters, but you might
want to adjust the element lengths to optimize gain and impedance.
I would strongly suggest that you model the antenna before putting it
up. The model would allow you to optimize gain, impedance and F/B. The
models are quite reliable. I'm sure that there must be someone on the
reflector who has already modeled this relatively common multi-band quad
arrangement. If not, I'll be happy to do so.
I can't comment on the T200-2 cores etc., but I use commercially
available W2DU type baluns here and they appear to work fine.
73, Joe
K2XX
P.S. Please give my 73 to the folks at RAST.
Martin Sole wrote:
> It's that time of year again. With the better weather now here and a
> little time to play I need to get outside and do some much needed
> antenna work. Way back during the stormy season my 2 el lightning bolt
> quad lost its 20m driven element, just snapped apart at a corner joint
> which if you ever tried that with aluminium welding wire you'll know is
> not an easy think to do so I guess it was a weak spot brought on by
> several build ups and take downs. We dropped the quad to a workable
> height on the tower and I currently have a C3 on the top but it is
> evidently no substitute for the quad so I'm keen to get it back up.
>
> I have decided to make a few modifications and would be interested in
> any suggestions before I get too far along with the job.
>
> First up I will be converting to separate feed for each of the 5 bands.
> There will be a remote switch box located on the boom at the driven
> element end.
> Feeders from the switch box will drop vertically from the boom end to
> each element.
> At the feed point a balun, T200-2 core with RG142, will interface each
> element to its feeder tail.
> Baluns will be housed in waterproofed (really) ABS boxes.
>
> A couple of questions:
>
> The Lightning Bolt quad has an 8' boom. From what I can determine this
> is quite wide spacing for 10 and 12 metres and performance might be
> improved if I were to re-locate one of the elements, just for these 2
> bands, such that the spacing is closer to 5 feet. I can do that by
> adding a pair of cross arms at the different spacing. Does that seem a
> likely improvement?
>
> Cebik mentions quite emphatically that to ensure decent performance from
> a 5 band quad, or any that involve multiple bands, it is necessary to
> ensure that the driven elements of the un-fed bands are shorted. In
> essence, when operating on 10m the 20m loop looks like a 2 wavelength
> loop and will have significant currents in it unless it is closed. It
> seems the only way to ensure this in all cases is by way of a
> switch/relay at the feed point. If this is a good idea I can arrange to
> do it quite simply. I have relays that should do the job, I am assuming
> that since they are open when the loop is fed and short when the loop is
> not fed these do not need to have great power handling. I have also
> thought about a similar relay switching method at the reflector to move
> the best F/B point. It probably sounds more complex in writing than I
> think it would turn out to be in practice. I will homebrew the switch
> box and provide separate contacts for the feed point shorting. Thoughts?
>
> I had thought of dropping a catenary rope from the boom end to support
> the feed points but now think the the combination of several feeders and
> any associated switch wiring should be sufficient support in itself. If
> I really should need some additional support what would you recommend?
> Dacron, phylistran and other wonder products are unobtanium here.
>
> Where do you buy T200-2 cores? I know I need to order these, just
> looking for a simple easy supply outlet.
>
>
> That's enough for now. Any ideas much appreciated.
>
> Martin, HS0ZED
>
>
>
>
>
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