[amsat-bb] Fixed elevation - how many elements?
kd6yam at amsat.org
Mon Jun 17 22:45:57 UTC 2019
Hi Juan Carlos,
Good question. There are a few reasons I'm leaning towards the DK7ZB
designs. No doubt others will have differing opinions, and I'd be more
than happy to hear them.
* The G0KSC antennas are based on an aluminum boom, whereas the DK7ZB
PVC designs are based on a PVC boom (duh!). For a given design (e.g.
5-el 2m), the boom is also slightly longer in the G0KSC design. Those
factors mean more weight. I want to keep weight to an absolute minimum
to avoid having to deal with stronger cross-boom, stronger mast,
stronger rotator, etc.
* Related to the aluminum boom, G0KSC makes quite a fuss about the
importance of insulators, about the importance of the insulator
design, and about the materials used to make the insulators. He even
includes links to commercial insulators. Going with a PVC boom, this
is a non-issue, since there's no need to insulate the elements from
* G0KSC provides a design that is built with 10mm diameter tubing.
DK7ZB provides measurements for several diameters, from 2.4mm to 8mm.
This gives me the option to use narrower (thus both lighter and
cheaper) material for the elements. If I build X-pols, that's twice as
many elements, so it matters.
* A while ago, I built an IOio antenna, so I'm quite familiar with
having to use a pipe bender to get the tubing bent correctly. It's not
easy (at least, it wasn't for me) to get the curves in exactly the
right places, trying to measure from the center of one curve to the
start of where to bend the next one. Also, with 3/16" tubing, which I
used for the IOio, it wasn't too big a deal to twist the tubing when I
didn't get two corners exactly parallel to each other. With the larger
tubing of the G0KSC design, though, that would be harder. (This is
evidenced by the fact that G0KSC even sells loop ends, noting that
"The most difficult part of building an LFA Yagi is bending the Loop
Ends and getting them right!".
So, in short, the DK7ZB designs seem, to me, simpler to build,
possibly less error-prone, and will give me very lightweight options,
even if I build X-pols.
On Mon, Jun 17, 2019 at 12:49 PM Juan Carlos Reig <ecoalfa5wa at gmail.com> wrote:
> Why not an LFA from G0ksc?
> I think is a good option
> 73's from EA5WA Juan Carlos
> En 17 jun. 2019, en 19:21, Martin Cooper via AMSAT-BB <amsat-bb at amsat.org> escribió:
>> Thanks very much, everyone, for all of the feedback and suggestions.
>> Much appreciated!
>> Now to figure out how to adjust the DK7ZB designs slightly for
>> satellite frequencies...
>> On Sun, Jun 16, 2019 at 7:32 AM Martin Cooper <kd6yam at amsat.org> wrote:
>>> I'm contemplating building a couple of yagis, possibly even crossed
>>> ones, and putting them up at fixed elevation (i.e. az-only rotator).
>>> What I'm unclear about is how many elements to consider.
>>> My location is really pretty dreadful for seeing sky. There's a big
>>> hill to the NE that restricts me to above 40 degrees in that
>>> direction. I'm in a single-storey house, with neighbouring two-storey
>>> houses to the NW, SW, and S. In between all that, there are other
>>> single-storey houses, and trees in most of the gaps between houses. I
>>> can't afford to put up a tower (and it could never get above the hill
>>> anyway), so I'm looking at not too much above my roofline.
>>> To at least get through the trees, I'd think more elements would be
>>> better. But more elements narrows the beam, and at fixed elevation,
>>> that, at some point, will restrict even further how much of the sky I
>>> can play with.
>>> I'm looking for thoughts on how many elements would be recommended,
>>> and how many would be too many, for a fixed elevation, both for VHF
>>> and UHF. Ultimately, I'd like to be able to get on the linears, so not
>>> limited to the FM sats. Any thoughts would be much appreciated.
>>> Thanks in advance.
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