Request EZNEC computation
 

..
Jerry Martes wrote:
"Ed" wrote in message
.92...
."

The answer to the question would seem to be that it would not work
very well just due to experiance. The swr would be off the charts and
a rig that had an internal tuner would not tune that kind of antenna
over that frequency range if the swr had to be under 3:1. Also the
loss in the coax due to the high swr would be very large..



Well, that was my original surmise of the situation. I was just
trying to have someone with expertize on EZNEC run the antenna at the
three frequencies listed in order to have the feedpoint SWR calculated
for each of those three frequencies. So far, no one, except Owen, has
come even close.


Ed, several years ago I had to find out why an almost identical
antenna system as you have discribed operating on similar frequencies
didnt work very well. System also used an ATU. Analysis showed data
very similar to the EZNEC values. I t was not practical to increase
the hiegth of the antenna so I went vertical using a 30ft fiberglass
whip, gov surplus. Also used inductors and capacitors switched in to
bring the antenna to resonance at the desired freq. Although we only
really used one the others had to be available. Although the ATU was
still used it was completely unneeded as only 3 frequencies were being
used seies inductance and capacitance selected by remotely controled
relays provide a match with the VSWR less than 1:.5 on any frequency .
Communication from NC to GA, FL, and DC were routinely possible with
100watts PEP SSB.


Jimmie

Request EZNEC computation
 

Ed, several years ago I had to find out why an almost identical
antenna system as you have discribed operating on similar frequencies
didnt work very well. System also used an ATU. Analysis showed data
very similar to the EZNEC values. I t was not practical to increase
the hiegth of the antenna so I went vertical using a 30ft fiberglass
whip, gov surplus. Also used inductors and capacitors switched in to
bring the antenna to resonance at the desired freq. Although we only
really used one the others had to be available. Although the ATU was
still used it was completely unneeded as only 3 frequencies were being
used seies inductance and capacitance selected by remotely controled
relays provide a match with the VSWR less than 1:.5 on any frequency .
Communication from NC to GA, FL, and DC were routinely possible with
100watts PEP SSB.


That could certainly be an option.... especially since this antenna is
on top of a government building. Aesthetics is an issue here.

However, the pre-existing antenna can easily be made to work well in
our situation simply by shortening it to resonance on 3975, and possibly
just adding traps or another dipole off the feedpoint for the other two
frequencies of operation. There is room. The reason for this silly
situation to begin with is just the misconception held by our ARES people
that the internal tuner of the FT900 could handle the mismatch.

Ed

Request EZNEC computation
 

Ed wrote in
. 192.196:

other two frequencies of operation. There is room. The reason for
this silly situation to begin with is just the misconception held by
our ARES people that the internal tuner of the FT900 could handle the
mismatch.

Ed, I worked up some numbers for you at 3.98MHz, and the numbers
demonstrated that the main cause of system inefficiency was coax loss (and
not the ATU).

If you are focussed on VSWR and bandwidth as you seem, you need a radiating
dummy load. They have certain merit in this application. Google for "TTFD
antenna" for an example.


Owen

Request EZNEC computation
 

Owen Duffy wrote in
:

Bushcomm's BBA-100 is designed for the type of application you have Ed:

http://www.bushcomm.info/index.php?a...od&productId=3

Request EZNEC computation
 

Ed, I worked up some numbers for you at 3.98MHz, and the numbers
demonstrated that the main cause of system inefficiency was coax loss
(and not the ATU).

If you are focussed on VSWR and bandwidth as you seem, you need a
radiating dummy load. They have certain merit in this application.
Google for "TTFD antenna" for an example.


As I indicated earlier, I am not communicating effectively with you!
Sorry if I have not been making myself clear, Owen. I am not "focused"
on those issues you mentioned, especially the ATU. I am aware of its
limitations and proper uses. I only wanted the data to help "prod" the
people in charge of the need to change the antenna system. SWR is a
figure that they recognize.

The Bushcomm looks interesting, but probably a little bit more wire
than we want to hang on this public building. The ARES group ONLY
operaties on three frequencies/bands. It is my opinion that we can very
easily add traps to the 75 foot halfwave antenna and shorten it at the
same time, giving us a resonant antenna with close to 50 ohm match on two
of the three operating frequencies. The third operating frequency can be
taken care of with another perpendicular dipole off the feedpoint. We
easily have end mounting points for both dipoles.

That should give us a much improved system for very little work.

Thanks, Owen.

Ed K7AAT

Request EZNEC computation
 

Ed wrote in
. 192.196:

The Bushcomm looks interesting, but probably a little bit more
wire
than we want to hang on this public building. The ARES group ONLY

The 'S' model is 66' overall, a little shorter than your current antenna.

These antennas are well suited to an application where the operator or
not a radio tech, or unfamiliar with the station, frequency agility is
needed, particularly rapid change in frequency with confidence that the
system is setup and ready to go, and where comms technology is second to
the job to be done.

No doubt there are similar things made in the USA, and I think Bushcomm
stuff is sold there (see Array Solutions).

BTW, someone suggested a vertical. Vertical's have very low gain at high
angles, so relatively unsuited to NVIS.

Owen

Request EZNEC computation
 

On 29 Jul 2007 22:25:04 GMT, Ed
wrote:

I only wanted the data to help "prod" the
people in charge of the need to change the antenna system. SWR is a
figure that they recognize.

Hi Ed,

After having heard the third chorus of:
I am not communicating effectively with you!
it is time to analyze why.

First and foremost, the correspondents here are focused on technical
solutions. You are seeking a political answer. The two do not
generally reside on the same page.

Technical correspondents here would reasonably expect that an
association composed of amateur radio operators would appreciate a
technical analysis to solve a technical problem. Owen and others have
provided that, and several have been specific to exactly the numbers
you have asked for. By and large, those numbers don't seem compelling
if your audience fails to appreciate the limitations of the internal
tuner's abilities.

SWR is not in and of itself loss. It does not always represent a quid
pro quo for efficiency. Your reliance on these numbers to sway a
group's rather myopic view of the problem gives all the appearance of
a magic beans solution suited for yokels.

Returning to the work we do here, there remain a number of missing
details to help us provide solutions, not political answers.

I've noted a number of correspondents have presumed this metal roof of
yours stretching out in an infinite plain. Clearly no such roof
exists. However, I have seen no further amplification of the details
by you. At a minimum, and as a gesture of good will towards those you
ask favors of, it would be useful to know the size of that roof, if it
is flat (or reasonably so); how high it is, and how high the antenna
is above earth (I presume you have only described how high the antenna
is above the roof).

These parameters have a bearing on the results of the EZNEC
computation you ask for. The metal roof has no particular effect in
terms of loss, and hence doesn't particularly effect SWR if resonance
is not perturbed. However, the proximity of earth does tend to
broaden bandwidth and to lower resistance of dipoles (shunt resistance
doing the job in both situations). This does impact EZNEC
computations you ask for.

Insofar as actually remedying the problem, that too has a technical
solution easily achieved courtesy of your metal roof. You simply
convert the dipole into an Inverted F. Consult:
http://www.qsl.net/kb7qhc/antenna/In...%20F/index.htm

Its discussion is couched in terms of UHF, but HF is merely a matter
of scaling. Also, you don't strictly need the wide, bent radiator as
single wires will do the same job to a narrower bandwidth (or you can
build a skeleton version as illustrated).

The Inverted F is suited to a long low horizontal section. This is
something that already exists and is probably driven by the lack of
higher supports. The metal roof conforms to the expected metal ground
conditions (UHF would make this simple to supply, you already have it
handy). The amount of wire is less than you already have, so no new
burden in that regard. The need for NVIS operation is clearly
supported. The design match obviates the transmission line losses
driven through the roof by existing mismatches.

73's
Richard Clark, KB7QHC

Request EZNEC computation
 

Richard,

Everything you said makes sense to me, but as I keep trying to convey,
accuracy is not necessarily needed in my particular situation.

The very basic issue here is the operation of a halfwave dipole
antenna, (resonant at 6.2 MHz), by this orgaznization and expected to
operate properly through over 100 feet of RG8 on those three specific
frequencies I mentioned. It is their expectation that the internal tuner
of the radio will handle any mis-match. Losses in the coax due to SWR do
not seem to occurr to them.

I was not seeking a "technical solution" to the antenna situation, as
I already have that... I would simply convert this antenna to resonance
on the operating frequencies. My intent in the original post was simply
to get some ball park figure for either antenna feedpoint impedance, or
SWR so that I could finish the loss calculations in the coax.,... I
certainly didn't intend to stir you guys up with a whole raft of
necessary details that would refine the data, but not really change it
in a way that would be significant to this particular situation!

I think I need to just haul my Wattmeter up to the feedpoint and
measure the loss for myself! Access to this roof is somewhat difficult,
but not impossible. It Certainly will provide the info I need to
convince these guys.

As far as the roof goes, it is a FLAT rubber covered metal roof on a
large 1 story building.... 14 feet above ground. The roof extends for
at least 50 feet beyond the ends of the antenna, and appears to be well
grounded at a number of points.

I really hope you all do realize I appreciate your responses... while
my replies to you may "appear" to be unthankful, that is far from the
case. I do read all that you discuss and learn much. Thank you.


Ed ( sorry for the top post - thought I'd leave Richard's post
below for reference.)





Richard Clark wrote in
:

On 29 Jul 2007 22:25:04 GMT, Ed
wrote:

I only wanted the data to help "prod" the
people in charge of the need to change the antenna system. SWR is a
figure that they recognize.

Hi Ed,

After having heard the third chorus of:
I am not communicating effectively with you!
it is time to analyze why.

First and foremost, the correspondents here are focused on technical
solutions. You are seeking a political answer. The two do not
generally reside on the same page.

Technical correspondents here would reasonably expect that an
association composed of amateur radio operators would appreciate a
technical analysis to solve a technical problem. Owen and others have
provided that, and several have been specific to exactly the numbers
you have asked for. By and large, those numbers don't seem compelling
if your audience fails to appreciate the limitations of the internal
tuner's abilities.

SWR is not in and of itself loss. It does not always represent a quid
pro quo for efficiency. Your reliance on these numbers to sway a
group's rather myopic view of the problem gives all the appearance of
a magic beans solution suited for yokels.

Returning to the work we do here, there remain a number of missing
details to help us provide solutions, not political answers.

I've noted a number of correspondents have presumed this metal roof of
yours stretching out in an infinite plain. Clearly no such roof
exists. However, I have seen no further amplification of the details
by you. At a minimum, and as a gesture of good will towards those you
ask favors of, it would be useful to know the size of that roof, if it
is flat (or reasonably so); how high it is, and how high the antenna
is above earth (I presume you have only described how high the antenna
is above the roof).

These parameters have a bearing on the results of the EZNEC
computation you ask for. The metal roof has no particular effect in
terms of loss, and hence doesn't particularly effect SWR if resonance
is not perturbed. However, the proximity of earth does tend to
broaden bandwidth and to lower resistance of dipoles (shunt resistance
doing the job in both situations). This does impact EZNEC
computations you ask for.

Insofar as actually remedying the problem, that too has a technical
solution easily achieved courtesy of your metal roof. You simply
convert the dipole into an Inverted F. Consult:
http://www.qsl.net/kb7qhc/antenna/In...%20F/index.htm

Its discussion is couched in terms of UHF, but HF is merely a matter
of scaling. Also, you don't strictly need the wide, bent radiator as
single wires will do the same job to a narrower bandwidth (or you can
build a skeleton version as illustrated).

The Inverted F is suited to a long low horizontal section. This is
something that already exists and is probably driven by the lack of
higher supports. The metal roof conforms to the expected metal ground
conditions (UHF would make this simple to supply, you already have it
handy). The amount of wire is less than you already have, so no new
burden in that regard. The need for NVIS operation is clearly
supported. The design match obviates the transmission line losses
driven through the roof by existing mismatches.

73's
Richard Clark, KB7QHC

Request EZNEC computation
 

In article . 196,
Ed wrote:

As far as the roof goes, it is a FLAT rubber covered metal roof on a
large 1 story building.... 14 feet above ground. The roof extends for
at least 50 feet beyond the ends of the antenna, and appears to be well
grounded at a number of points.

Ed,

I hope that once you figure out a more efficient matching system
(traps, or feedpoint coils, or whatever) and get your TX power up to
where you want it to be, you don't regret having succeeded :-)

Our ARES/RACES group has its shack located in our city's police-and-
fire admin building, which also hosts the city's 911 dispatch center.
We have a 40/80-meter trap dipole, mounted perhaps 20' above the roof,
running the length of the building.

We haven't had any matching problems with it.

What we *have* had, is a severe problem with RF incursion into the
building's phone system, including the 911 lines. At first we thought
it might be due to RF on the outside of the coax feedline (which runs
near the phone wiring) or RF leakage from the shack into the phone
wiring in the closet next-door. Feedline chokes didn't help,
unfortunately.

In the end, we ran a test in which we opened up the feedline near the
base of the central antenna tower (disconnecting the antenna from the
shack and from all of the in-building coax), and transmitted through
the antenna using a portable HF radio on battery power. The phone
lines still picked up the signal. Grounding or "floating" the HF
radio made no difference at all. The problem is apparently due to RF
near-field pickup by the phone wiring.

Unfortunately, the city comms folks weren't interested in making the
large effort (and handout of cash) needed to actually diagnose the
deficiencies in their in-house phone system, and install filters and
ferrites on the lines to choke off the RF pickup. Our only practical
solution was to limit our TX power to below the point which causes RF
incursion (varies by band... 20 watts usually seems to be safe).

I can't really blame the city folks, as our HF setup is used only
infrequently and is about a sixth-level backup to the city's other
communication systems. If we were operating independently, out of a
non-city-owned building using non-city equipment, we could make the
case that the incursion is their problem, not ours (according to the
FCC), but since they own the building and the gear we can't do that.

If you're lucky, your increased TX power won't cause you any such
problems... but you might want to consider the implications of having
this sort of incursion problem.

--
Dave Platt AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

Request EZNEC computation
 

Richard Clark wrote in
:

On 29 Jul 2007 22:25:04 GMT, Ed
wrote:

I only wanted the data to help "prod" the
people in charge of the need to change the antenna system. SWR is a
figure that they recognize.

Hi Ed,

After having heard the third chorus of:
I am not communicating effectively with you!
it is time to analyze why.

Hi Richard,

This all seems much ado about nothing since it emerges that the real
objective is to inform a committe in VSWR speak that the VSWR at the
antenna side of the radio's ATU is more than the specified 3:1
capability.

It seems to me that could be done very easily by measurement, and all the
uncertainty of models that resemble to some greater or lesser extent is
not only unnecessary, but introduces significant error due to the
assumptions used to simplify the model.

It seems that the reduction of the modelled feed point impedance of a
centre fed dipole over an infinite metal plane using an efficient and
effective balun to a single un-qualified metric, the holy VSWR, is a bit
of a fraud.

There is a certain element of "it doesn't matter if it is accurate or
valid, so long as it is compelling".

Owen