Parasitic question
 

With a 1/4wl vertical (mounted close to ground level) and you add a parasitic
(either a reflector or director) do you just ground mount it or do you need
radials or does it depend upon the ground conditions?

I noticed in running a few simple examples on EZNEC that the impedance drops
with the number of parasitics and the spacing. for example the 1/4wl alone will
be close to 50 ohms and with a parasitic the impedance will drop to less than
half in some arrangements (especially with 0.15wl spacing). Is this due to the
interactions of the driven and parasitic element?

By the way I was quite impressed by the potential F/B ratio and almost
cardioid pattern with a reflector and director added

Alan KB7MBI
Woodinville, WA

On 29 Oct 2003 16:20:25 GMT, (ARDUJENSKI) wrote:

With a 1/4wl vertical (mounted close to ground level) and you add a parasitic
(either a reflector or director) do you just ground mount it or do you need
radials or does it depend upon the ground conditions?

I noticed in running a few simple examples on EZNEC that the impedance drops
with the number of parasitics and the spacing. for example the 1/4wl alone will
be close to 50 ohms and with a parasitic the impedance will drop to less than
half in some arrangements (especially with 0.15wl spacing). Is this due to the
interactions of the driven and parasitic element?

By the way I was quite impressed by the potential F/B ratio and almost
cardioid pattern with a reflector and director added

Alan KB7MBI
Woodinville, WA

Hi Alan,

The parasitic is simply an antenna that is not directly driven. In
other words, to be efficient it has to conform to all the requirements
of the active element. In more words, it also follows all the
variables that affect the active element.

Do you ground mount it w/wo radials? Yes/no. Symmetry usually brings
its reward, but there are certainly a lot of variations that maintain
symmetry without being exactly identical. Further, the addition of
the parasitic inevitably upsets the original configuration (sometimes
to the benefit of the user) through interactions. Why? Just like
paralleling resistors changes the load R. In this case, the fields do
the job of connecting up instead of wire. Here in Woodinville, trees
will do the same thing.

The F/B thing will vary with coupling (distance/wavelength) and
efficiency (back to ground conditions). Simple examples abound in the
AM band. Examine the slough south of Bellevue near the I-90
connection to Mercer Island.

73's
Richard Clark, KB7QHC

The ground electrodes or radials of ground-mounted vertical radiators,
parasitic or directly-fed, reflectors or directors, must be given just as
much loving care and attention as the primary vertical radiator.

On 29 Oct 2003 20:28:07 GMT, (ARDUJENSKI) wrote:

Richard and Reg
Thanks for the feedback. Let me ask a question that goes hand in hand with the
initial querry. When you set up the vertical and its parasitic how do you tune
them for either max gain or max F/B ratio? You can set the height differences
by plus or minus 5 percent and space between 0.15 and 0.20 wl but this will get
you in the ballpark. I had heard to install a variable cap at the base to fine
tune the arrangement but how do you tune it? With a second person and a field
strength meter and either tune for max fwd signal or min signal off the back?

Hi Alan,

I would suggest you return to your modeling with EZNEC for a first
pass approximation. As for the final tune, I cannot see how you can
escape the test of the model without some field work. Max Gain and
best F/B are not always achieved with the same solution; so you may
wish to commit to which is more important. You may also want to
investigate how to reverse the pattern (switchable driven element).

73's
Richard Clark, KB7QHC

Richard
I agree in that you need to chose either focusing on F/B or forward gain, just
like the YAGI. I guess from a practical point of view how is this accomplished?
I am not quite sure how far the field strength reading need to be to be of
value. I guess a little trial and error is in order here.

As you noted computers will get you in the ballpark but the real proof is in
actual testing which takes into account other variables.

I am a little surprised that there is not quite a bit of literature on this, or
at least not what I have found (tuning parasitics). Maybe some reading up on
the YAGI may help shed some light

As an aside I have worked on arrangements that do not employ radials such as
the inverted half square for 20 and 40m. This is where you have two 1/4wl
verticals elevated about 0.05wl and spaced about 1/2wl and the phasing line is
at the bottom vs the top. This enables you to have a rotatable half square and
get about 2 S-unit rejection off the ends (just like the calculations). The
nice thing is portability and not radial dependent. I got off the subject.

I wil try the strength meter at varying distances to see if it makes a
difference.

Thanks---Alan KB7MBI

On 30 Oct 2003 06:27:42 GMT, (ARDUJENSKI) wrote:

Richard
I agree in that you need to chose either focusing on F/B or forward gain, just
like the YAGI. I guess from a practical point of view how is this accomplished?
I am not quite sure how far the field strength reading need to be to be of
value. I guess a little trial and error is in order here.

As you noted computers will get you in the ballpark but the real proof is in
actual testing which takes into account other variables.

I am a little surprised that there is not quite a bit of literature on this, or
at least not what I have found (tuning parasitics). Maybe some reading up on
the YAGI may help shed some light

As an aside I have worked on arrangements that do not employ radials such as
the inverted half square for 20 and 40m. This is where you have two 1/4wl
verticals elevated about 0.05wl and spaced about 1/2wl and the phasing line is
at the bottom vs the top. This enables you to have a rotatable half square and
get about 2 S-unit rejection off the ends (just like the calculations). The
nice thing is portability and not radial dependent. I got off the subject.

I wil try the strength meter at varying distances to see if it makes a
difference.

Thanks---Alan KB7MBI

Hi Alan,

Comparisons to yagis is intriguing, but likely going to be harder to
obtain equal results barring considerable ground development.
However, your comments about elevated structures brings back the
symmetry I discussed earlier and their equivalents. The relation of
director/reflector is bound to be a useful paradigm, and using tuning
to enforce that relationship would pay off. Just remember it is all
about phase; and phase is a product of both distance and electrical
delay within the resonant structures. These phase relationships are
more quickly experimented with in the computer to answer your question
about gain/F/B; but in all reality, with just two elements the F/B is
probably going to yield more dramatic results. In other words, the
best you will do by gain would be 3dB, but the F/B may easily find
10dB variation or more.

I think Roy's work on phasing antennas is in one of the ARRL Antenna
Compendiums and it should be sought as a source for you to reference
in these matters.

73's
Richard Clark, KB7QHC

Richard
I will check out the Compendiums. Regarding the ground affect and the YAGI vs
parasitic verticals, once this is figured out I would like to take a Moxon
Rectangle and rotate it vertically. On this axis it would be easier to rotate
it but need to check out the same factors you mentioned here.

As an aside, we all know the ground plays an important role in the antenna
performance yet it seems there is very little effort on behalf of the amatures
to measure actual ground conditions near and far field. Getting back to the
parasitic radials, TUNING ther verticals at least allows us to compensate for
this unknown.

I know in Reg's antenna programs I generally assume AVERAGE ground conditions
or about 20 ohms to be on the conservative side. What would be nice is some
sort of set up that you could tune the antennas by use of an audio sound. It
seems in theory that you could connect a receiver to the vertical driven
element and have a signal source a few hundred yards away or more and tune the
forward for max volume and a signal from behind for min volume and strike a
happy medium....Alan

On 30 Oct 2003 08:43:23 GMT, (ARDUJENSKI) wrote:
Hi Alan,

As an aside, we all know the ground plays an important role in the antenna
performance yet it seems there is very little effort on behalf of the amatures
to measure actual ground conditions near and far field. Getting back to the
parasitic radials, TUNING ther verticals at least allows us to compensate for
this unknown.

Tuning and ground should have no relation except in Q. The tuning is
for the benefit of variable phase delay and the proximity of ground is
going to blur that distinction causing the F/B to back fill the null.


I know in Reg's antenna programs I generally assume AVERAGE ground conditions
or about 20 ohms to be on the conservative side. What would be nice is some
sort of set up that you could tune the antennas by use of an audio sound. It
seems in theory that you could connect a receiver to the vertical driven
element and have a signal source a few hundred yards away or more and tune the
forward for max volume and a signal from behind for min volume and strike a
happy medium....Alan

That sounds rather complex. How are you going to hear the sounds,
loudspeakers? Won't the neighbors mind? Remember, if you are close
enough to hear, you are close enough to upset the tuning. I suppose
you are trying to accomplish this solo, but keep in mind that near
fields (those within 1 wavelength radius of the elements at a minimum,
3-5 wavelengths modestly, 10 conservatively) do not geometrically
represent the launch characteristics observed in the far field (which
dominates all our activities).

73's
Richard Clark, KB7QHC