[KL7AA] Working Wednesdays
Jim Wiley
jwiley at gci.net
Thu May 7 02:30:13 EDT 2015
Adam -
Just a few follow up notes concerning antennas, and the Buddistick (or
Buddipole) in particular. While it is true that you can develop some
useful information about adjustments to the antenna by making note of
parameters such as coil tap positions, those measurements and notes will
only be repeatable if the exact same conditions are repeated at another
location, which may be somewhat of a crap-shoot. Taking measurements
and counting coil taps will get you "in the ballpark" as a starting
point, differences in ground performance and other local variables will
almost certainly require additional adjustment when you get to a final
operating location.
That is not all a bad thing, if you understand how to compensate for
these differences. First thing to remember is that any type of loaded
(shortened) antenna will have a narrower operating bandwidth and
efficiency that of a "full size" antenna. Next, and again I stress this
point, SWR in and of itself is not particularly important, as long as
you can achieve a "match" at the transmitter output connection.
Following that, keep in mind that reducing SWR below a certain point is
somewhat of an exercise in futility. As a rule if you can reach an
indicated SWR of less than 2:1 you are good to go. Reducing SWR below
that value may make you feel good, but it will do little to improve your
outgoing signal.
I mentioned that any shortened antenna will experience a reduced
operating bandwidth. What does this mean? It simply means that you
will need to re-tune the antenna system more frequently as you change
frequencies within a band, and in some cases that retuning may be
required fairly often. So, the question becomes how do you tell when
those limits have been reached, or exceeded? Here is where some basic
instrumentation becomes useful.
First consideration: Your transmitter probably has a solid-state final
amplifier stage, which means it is designed to match a 50-ohm antenna,
and that it is not adjustable. No problem, but you need to be ready to
deal with conditions that are not "perfect", and so some sort of
adjustable matching system is called for.
Second consideration: You are talking about a "Buddipole" antenna, which
is basically an antenna with several "tap" points that can be selected
for various HF amateur bands. It is usually erected in the dipole
configuration (which by the way is more accurately called a "doublet"
antenna), but can be assembled as a vertical monopole antenna by using
only one half of the kit (thus becoming a Buddistick). Tuning the
antenna is accomplished by selecting tap points on the included
"loading" coils.
Third consideration: You are mostly interested in HF operations. While
the Buddistick / Buddipole antenna can operate on the 6 and 2 meter
bands, tuning and adjustment for those bands are a different game
altogether.
So, given all of the above, let's concentrate on HF operation. While a
Buddistick / Buddipole antenna can be erected so it is relatively high
(70 feet or more) above ground, it more commonly is set up fairly close
to the Earth, meaning anywhere from 5 to 20 feet above ground. This low
mounting configuration creates two significant effects: (1) the
radiation resistance and thus the tuning point where it is "matched" to
the transmitter will fluctuate quite a bit depending on actual height
above ground, and (2) for almost all HF bands, the radiation will be
virtually straight up. This second thing is not all bad, and in fact can
be quite beneficial, depending on what you are attempting to accomplish.
However, the antenna can be tilted so one end is higher (or lower) than
the other, and depending one the degree of tilt, the main lobe of signal
radiation can be aimed more toward the horizon than the zenith. Again,
the degree of tilt will affect the tuning of the system. Low angle
signals are more effective in contacting distant stations, particularly
on the 20-meter and higher frequency bands.
The "high angle" radiation is useful if you are attempting to reach
close-in stations (stations within Alaska) on the 80-meter and
40-meter bands. The radiation goes essentially straight up until it
encounters the ionosphere, at which point it is reflected fairly
uniformly to all stations that are within a circular radius of perhaps
500 to 700 miles. This is exactly what you want for participation in a
local area traffic net or for area-wide disaster relief type
communications.
So, somewhere in here you need to consider some sort of
instrumentation. There are three types of instruments that will be of
use here. They are: (1) A "SWR Bridge". This is a relatively simple
and inexpensive instrument that will give you an indication of forward
and reflected power. Such an instrument will do the job, but it
requires frequent readjustment and you change bands or even frequencies
within a band. (2) A directional wattmeter. This instrument can be
built with either a single meter or a dual meter that allows
simultaneous observation of both forward and reflected power. The dual
meter version is much easier to use, and is recommended. Among other
things, it is relatively immune from frequency vs. sensitivity issues.
(3) An antenna analyzer. This instrument gives the most comprehensive
information, but has the disadvantage of requiring that the antenna be
disconnected from the transmitter and connected to the analyzer each and
every time it is used, and then at the conclusion of a measurement the
process much be reversed, and unless you are using an external power
source, the analyzer will "eat" batteries fairly quickly.
The first two instruments are intended to be left connected to the feed
line between the transmitter and antenna, and can be used while
transmitting a signal, and thus are much easier to use in every-day
operation. They are also self-powered, and do not require batteries.
It is very possible to home-build (construct the devices yourself)
either the SWR meter or the directional wattmeter.
The third device, the antenna analyzer, is probably beyond the skill
level of even fairly advanced amateurs. Not to say that you could never
build one, but it is not easy, and calibration could be a problem.
All of these instruments can be purchased completely assembled and ready
to use. There is even a very well engineered kit that combines an
automatically ranging wattmeter and SWR bridge in one instrument. It
is, however, not cheap ($100.00). But, it is a _very_ good instrument,
and an easy to build "one evening" kit. I have one.
But, if you are like most of us, there is satisfaction in building some
of your own station apparatus from parts you have gathered, and the cost
is obviously much less. If you are interested in the "build it
yourself" approach, let me know. I have most if not all of the parts
(and tools) you will need, all you have to do is invest some time and a
bit of thought. As it turns out, I happen to have an ample supply of
small panel meters that can be used, and they even have the appropriate
dial scales already on place.
I hope this has been helpful, and as I said before, this only touches
the surface of the subject. But, if you want to know more, you can
contact me and we can continue the discussion. I can be reached as
VE at kl7aa.net or kl7cc at arrl.net
Once you have some instrumentation in hand, then you can think about how
you are going to make some sort of adjustable "antenna tuner". It is
true that you can do without one and just do the matching by fiddling
with coil taps, bit trust me, that soon gets pretty old pretty fast.
Here is something to think about: it does not matter where the "antenna
tuning" is done, as long as the eventual result makes for an antenna
that can accept forward power from the transmitter while minimizing
reflected power. All the tuning can be done at one place, or it can be
distributed between two (or more) places. The final result will be the
same.
What that means is that you can use the coil taps on the antenna to get
"close" and make the final adjustments via some sort of antenna tuner
(also called an antenna coupler). Like the SWR meter and
dual-wattmeter referred to previously, these components can be purchased
completely assembled and ready to use, or built yourself from off the
shelf components. Again, I have most if not all of the needed parts in
my "junk box".
73
- Jim, KL7CC
On 5/6/2015 5:56 PM, Adam Booth wrote:
> Hello All,
>
> I am curious how "Working Wednesdays" well... work. Are they on a
> specific Wednesday of each month or every Wednesday?
>
> I have a Buddistick that is incredibly finicky to tune without an
> antenna analyzer (which I don't have) so I was hoping to bring it by
> the clubhouse sometime and maybe borrow an analyzer so I can tap the
> coil in the specific points I need for the bands I'm interested in. I
> also need the analyzer to determine and mark my counterpoise lengths.
> I have also never been to the club house so it would be interesting to
> check out.
>
> So far I have been tuning by ear and that has worked just okay, I
> would like to get it tuned to right where it needs to be for maximum
> efficiency. I made my first contact yesterday evening to Japan over
> CW and I am absolutely hooked.
>
> Thanks and take care!
>
> Adam Booth
> KL3TX
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