Buck wrote:
I tried it again and got the picture. It appears that the 450 ohm
twin lead sections can match any impedance between 20 and 100 ohms
given the proper length.

Not only them, but *any* impedance that causes a 450 ohm
SWR between 4.5:1 and 18:1. That's an infinite number of
impedances (every impedance within the green area) and includes
virtually all the actual impedances encountered when the dipole
is 1/2WL on the lowest frequency of operation. For instance,
the feedpoint impedance of an 80m dipole usually falls into this
green range when used on 40m (and any other HF band) even though
the feedpoint impedance might be 5000 ohms on 40m.
--
73, Cecil http://www.qsl.net/w5dxp


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Buck wrote:
Ok, just to reiterate so I know I understand (and you can see if I
don't), If I feed a 20 meter dipole cut for my favorite frequency, I
can feed the radio with exactly 1/2 wave of 450 ohm feedline and it
will match the radio?

Actually, any integer multiple of 1/2 wavelengths will work

Of course, coax will make a better match (with
less loss?)

Amazingly, not necessarily true. Better match? Yes. Less loss?
Not necessarily. Ladder-line with an SWR of 9:1 may actually
have less loss than coax with an SWR of 1:1.

Depends upon the type of coax and the type of ladder-line.

For instance, at 10 MHz, RG174 has a matched line loss of 4 dB
per 100 ft. while 600 ohm open-wire line has a matched line loss
of about 0.075 dB, so small I have trouble reading it on the chart.
Even adding the additional losses due to SWR, the open-wire line
is far, far below the RG174 matched line losses.

Now, if I understand, the principle of the 130 foot dipole matching
the rig is based on the length of 450 ohm twin-lead. The twin lead is
always 1/2 WL long.

Nope. The twin lead is whatever length it takes to resonate
the antenna system plus an integer multiple of 1/2WL. Correct
this concept first before drawing additional conclusions.
--
73, Cecil http://www.qsl.net/w5dxp


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On Tue, 14 Jun 2005 12:38:31 -0500, Cecil Moore
wrote:

Buck wrote:
This is interesting. I remember reading where the G5RV originated as
a gain dipole for 20 meters. I hadn't looked at it as a 1.5wl dipole.
I have always used 1.5wl dipoles for 15 (40 meters) and had low SWR on
the 15 meter band without a matching network. Am i missing something
or is the coax stealing enough power to reduce the SWR?

Bingo! Your 15m SWR at the 40m dipole feedpoint is probably
above 2:1. Feedline losses both ways probably reduce it to
a value tolerated by your transmitter. If you replace the
coax with an integer multiple of 15m halfwavelengths of
ladder-line to reduce losses, your transmitter may object.

I have inadvertently stepped upon an unpublished article with
my posting. I apologize and will bow out.


I find it interesting as it has always been flat. Even better on 15
than 40 most of the time. 2:1 isn't terribly bad, most transmitters
allow it, or close to it before reducing power. What is the impedance
of that?

actually I'll try it on eznec, that's one antenna I can model.

Thanks for the comments.

what's the unpublished article? the G5RV? I don't remember where I
have read it, but then as we have seen here many times, not everything
one reads on the internet (or in the newspaper, for that matter) is
accurate.

DOn't shy away. If you know differently, i am open to hearing it.




--
73 for now
Buck
N4PGW

On Tue, 14 Jun 2005 13:27:49 -0500, Cecil Moore
wrote:

Now, if I understand, the principle of the 130 foot dipole matching
the rig is based on the length of 450 ohm twin-lead. The twin lead is
always 1/2 WL long.

Nope. The twin lead is whatever length it takes to resonate
the antenna system plus an integer multiple of 1/2WL. Correct
this concept first before drawing additional conclusions.

I think I meant a 1/2 wl interval, but apparently that is wrong too.

If I have an antenna at impedance A, I would first run a length of 450
ohm twin lead for X length which will create the matching impedance
and then any integer multiple of a half wave after that.

A and X being unknown.


--
73 for now
Buck
N4PGW

On Tue, 14 Jun 2005 12:59:42 -0500, Cecil Moore
wrote:

Buck wrote:

W5DXP wrote:
What you are missing is that first piece of matching section
which can be any length less than 1/2WL. After that first
piece of matching section, you could indeed switch to a
different characteristic impedance. Congratulations, you
have just invented the G5RV. :-)

Fred, I believe that you and Cecil are 180 degrees out of phase. Did
someone make a type-O?

Well, I made an English language blunder as I was editing my posting.
I should have dropped the which above. The first piece of matching
section, which can be any length less than 1/2WL, brings the impedance
looking into it to a purely resistive value, i.e. a resonant value,
i.e. a current maximum point. So the first section of matching is the
most important. After that, any Z0 could be used, but *only on that
particular band*. That last *constraint* may be what you are missing
(if you are missing anything at all). I am not aware of any phase
difference between Fred and me.


Fred says the matching section needs to be any length more than a 1/2
wave and you say it is less than a 1/2 wave. This is my confusion...


--
73 for now
Buck
N4PGW

Buck wrote:
what's the unpublished article? the G5RV? I don't remember where I
have read it, ...

It's somebody else's unpublished article and if it's
"unpublished", of course you haven't read it.
--
73, Cecil http://www.qsl.net/w5dxp

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On Tue, 14 Jun 2005 13:13:01 -0400, Buck wrote:

[snip]

I am not sure what I just did or saw. I had several colored graphs
charted and one gray. The gray didn't seem to move around, but the
colored ones sure did. I am afraid I didn't see what you were trying
to show me. I believe the colors were the different feed lines,
capacitors, etc used in the feedline shown. I tried removing them and
only leaving the feedline I chose to use. I don't know if I did
something wrong or not, but the gray line appeared to be the antenna
and it never appeared to move. The feedline markers moved radically.

I guess you're looking at the Smith chart in XLZIZL. What you are
seeing is correct. The gray trace is the load impedance and absent
changes in the antenna it will be fixed. If you look right to the
legend you will see what each of the colors represents.

So if you've modeled something in EZNEC and read the resulting data
file into ZLZIZL.xls and did a "Refresh and show Smith chart" you will
see the data plotted as the "Load"

Let's do a little (bit long) experiment that will clarify this (I
hope) and also demonstrate some of what happens in a matching network.

1. Open EZNEC and open the file "BYdipole.ez".

2. Go to "Setups" (at the top of the window) and select "Frequency
Sweep."

3. When the pane opens, select "On" and type in 14.35 for the "Stop
Frequency" and .05 for the "Frequency Step."

4. Under output. check the box "Microsmith Files" and for file name
type "g5rv" and hit Ok.

5. Go to the "Wires" menu and change the Y value for End 1 to -51.
Change the Y value for End 2 to 51.

6. Do a "Save As" and type "g5rv".

7. Perform a Freq Swp. (Bottom left button).

8. Open XLZIZL, clear any old data or networks and read the g5rv.gam
file.

9. Press "Refresh and show Smith chart."

10. The gray trace is the plotted impedance of the feedpoint of the
102' dipole over the 20-meter band.

11. Now at this point Varney (G5RV) recommended a "matching section"
of 34' of open wire line, followed by a run of 72 ohm line to the
shack. (If you want to see how bad this idea was, you can experiment
with adding the lines in the Network dialog)

12. We are going to do it the "modern" way, by using ladderline and a
tuner.

13. In the Network definition dialog, select element position 5 and
set the type to "Transmission Line", select "Wireman 554" and set the
length to 50'. Set the element and close the pane.

14. Press the "Refresh and show Smith chart." The red trace shows the
impedance at the input end of the 50' transmission line. Since the
desired target is the center of the chart, we can seen that this was a
negative "improvement."

Note: I didn't contrive this feeder length, it was just a round
number that seemed like an average situational value that follows the
"conventional wisdom" that says, "Use a length long enough to reach
from the antenna to the rig."

15. This is what we're stuck with if we decide to use ladderline and
a tuner, so let's muddle on.

16. In the ZIZL worksheet at row 38 there are some boxes for
Component Q. Enter 500 for Qu(C), 250 for Qu(L) and 14 for Q ref
freq.

17. Now let's explore a slick feature of this program and design a
matching network. Highlight the cells F106 and F107.

18. Look at cell I43 and press "Copy Selected R,X to T-Pi-L".

19. Focus will transfer to the T-Pi-L worksheet and a query window
may open asking whether you want to copy the Q values from the ZIZL
sheet. Answer Yes. You will no doubt get a second advisory pane that
says that frequencies differ. Answer "Ok". Cell H10 will be
selected. Type 14.2 into it and press Enter or Tab to input the data.

20. We are now presented with four built-in matching network
solutions. The most used topography for commercial tuners these days
is the "High Pass T." (Option 1) So we will use it. You can press
"Schematic" and see the configuration. Press the "Copy to FT/Solver"
button. (Cell B24)

21. The last action took the precalculated values and moved them to
the Fine Tune / Solver Area. For now, we won't explore the attributes
of this feature. Press the "Copy to ZIZL 1-3" button. (Cell K26)

22. Focus will return to the ZIZL sheet and a query pane will open.
Answer "No." If you look at the network definition table area you can
note that there are now three components in addition to the
transmission line in evidence.

23. Press "Refresh and show Smith chart."

24. Focus returns to the Smith chart where a wealth of information is
presented. In the upper right corner there is a box that shows
exactly what the feed system consists of. The "Plot Segments" box
shows the legend for each trace and what the effect of each network
component is. The informational box to the left will show the
parameters of any data point on any trace by just double clicking the
point.

Of course, the trace for Plot Segment 1, the input series C, is what
we are after. All but the 14.0 MHz point fall inside a 2:1 SWR
circle. Let's see if we can fix that.

25. Press the "Tune / Set" button (lower right corner). Long years
of experience tell me to tune the input capacitor first, so that's
what we'll do, but this may not always be the case. In the pane that
opens, select, "1:Series C", which if all of the calculations are the
same, should equal 51.2 pF. Mouse down a bit and find the +-1% option
and select it. We are going to "tune" the input capacitor in 1%
steps. Click the "Increase" up-arrow three times. Voila! The SWR is
less than 2:1 over the whole 20-meter band with one set of
adjustments. With each click you could see the action of "tuning"
this component. Note that none of the other traces moved at all. All
we changed was the match between the input and the junction of the
input C and the shunt L. The match from the junction to the antenna
was unaffected.

Because this is getting a bit long-winded and off-topic, I'm going to
stop here and pick this up in a new thread.



I haven't thought a lot about the losses in ladder-type feedline, but
I do understand how the losses in coax can reduce High SWR by reducing
both the outgoing and incoming reflected signals in the coax. This
may make the antenna desirable to the rig, but it doesn't do any good
for the operator who is trying to reach or copy that weak signal.

Correct.

It
may well be that the Twin-lead has the same effect.

It does.

I often read that
one of the advantages of the twin-lead is that it can handle higher
SWR without the higher losses of coax.

I seem to have been seeing a lot about the technology of the twin-lead
tuning technology, but then I can't be sure it wasn't from the same
people here on this forum. I'll continue to look into it to see what
I might learn.

Some of the people here know what they are talking about... others I'm
not so sure. I'm unlocking the tool box, so you can figure it out
yourself.


I was able to get past whatever block I had on learning to use EZNEC,
so at least some good came from this discussion. Hopefully, I'll
learn more and can model this antenna and others I read about.

I'm sure you will.


Thanks very much for taking the time to address this with me.

No problem. The guys that really deserve the thanks are the geniuses
1: who developed the NEC engine, 2: Roy, W7EL, (among others) who put
a pretty face on it, and 3: Dan Maguire, AC6LA, who did the Excel
stuff.

And the absolutely amazing thing is, they give away their work! Is
this a great country or what?

Regards,

Wes N7WS

On Tue, 14 Jun 2005 12:30:48 -0500, Cecil Moore
wrote:

Buck wrote:
I seem to have been seeing a lot about the technology of the twin-lead
tuning technology, but then I can't be sure it wasn't from the same
people here on this forum. I'll continue to look into it to see what
I might learn.

The "twin-lead tuning technology" is based on the laws of
physics, not the approval of the gurus on this newsgroup.


I am not in a position to argue with either side on the issue. I am
just doing my best to learn. Here is one lesson I learned in 25 years
of ham radio, Either there is sufficient propagation for your station
or there is not. Without propagation, a kw amp in a 12 element beam
will not get you across the ocean, and if there is sufficient
propagation, one can make a contact with a dummy load (I saw this
demonstrated once on 75 meters).

Probably the things I remember most about antennas is what I learned
from the two Jims of Taylor Radio, a local antenna MFG company at the
time. "More technological advancements have been made in marketing
departments than in Research and Development."

I realize that there are several here who actively disagree and
discuss their disagreements with each other. I haven't seen any, so
far, that are all BS and have no knowledge to offer. I don't
criticize anyone, but I will ask, suggest, and offer opinions that I
hope will be either corrected or supported appropriately.

I have used experimental antennas that theoretically won't 'work' and
made contacts. One was a roll of 500 foot of wire. I unrolled an
estimated 1/4 wave of wire for 80 meters, on the first try it was
1.5:1 or less and I used it to check into the GA SSB net. I had a
weak signal, but it worked. I also hung an inverted V with a random
length of wire from each side (I don't remember the length, but they
were both the same length.) I connected one side of the feedline and
dipole to the left side of a 100ohm resistor and the other side of the
feedline and dipole to the other side of the resistor. My theory was
that the antenna would be a parallel circuit leaving the impedance
below 100 ohms. I don't know if my theory worked, but I did make
contacts. When I took down the antenna and replaced it with a shorter
dipole, I had a much better receiver than I had with the resistor.

I am always looking for that 'best' all-band hf antenna so I don't
have to change or tune, etc. One day I'll either find it or die
trying. Until then, I plan to enjoy learning more about antennas and
experimenting with them.

73 for now
Buck


--
73 for now
Buck
N4PGW

On Tue, 14 Jun 2005 17:58:44 -0400, Buck wrote:

[snip]

I have used experimental antennas that theoretically won't 'work' and
made contacts.

Me too.

When I was 16, and a newly minted Conditional class, my BC-342
receiver broke (I still have it and it's still broken...gotta fix that
someday). I had a Heathkit DX-100 transmitter, which covered 160
meters. So I took an old tube type car radio, which in those days
used a 262 KHz i-f and permeability tuning, and tweaked the inductors
so it would tune the 160-meter band. I acquired a 262 KHz crystal and
built an oscillator that I just let leak into the receiver for a bfo
so I could copy CW.

My bedroom (shack) had steel casement windows and I had removed a pane
and replaced it with a sheet of aluminum (no access to plastic in
those days or coax, I was poor) with some ceramic feed thru
insulators. I ran a single wire from the rig to near the top of a
Eucalyptus tree (talk about dying for an antenna). I don't remember
the length, maybe 100 feet, or the ground system; a single rod I
think.

My first and only contact with that setup was DX; a VE7. The logs for
those days are long gone so he's not in the current computer log. But
if he was I would have a grand total of five countries worked on 160;
about one every ten years [g].

Buck wrote:
. . .
I have used experimental antennas that theoretically won't 'work' and
made contacts. One was a roll of 500 foot of wire. I unrolled an
estimated 1/4 wave of wire for 80 meters, on the first try it was
1.5:1 or less and I used it to check into the GA SSB net. I had a
weak signal, but it worked. I also hung an inverted V with a random
length of wire from each side (I don't remember the length, but they
were both the same length.) I connected one side of the feedline and
dipole to the left side of a 100ohm resistor and the other side of the
feedline and dipole to the other side of the resistor. My theory was
that the antenna would be a parallel circuit leaving the impedance
below 100 ohms. I don't know if my theory worked, but I did make
contacts. When I took down the antenna and replaced it with a shorter
dipole, I had a much better receiver than I had with the resistor.


What theory is it which says those antennas won't "work", given that
your criterion for "working" is making contacts? I've made contacts on a
light bulb, worked Alaska from a Colorado basement on 20 meters with 50
watts of SSB using a dipole(*) draped around the room with no part
higher than ground level, worked New Hebrides on 40 meter CW running 1.5
watts to a bent attic dipole(**) 16 feet off the ground, and JA running
8 watts on 40 meter CW with a base loaded CB whip bumper mounted on a
VW. These aren't exceptional -- every ham who's operated for some time
has a handful of similar stories. All those antennas "worked". Any
theory which declares they can't is wrong, and should be discarded. No
theory I know says they can't.

(*) The feedline was 72 ohm twinlead, with one conductor stuck into the
rig's SO-239 center pin and the other clipped onto the rig's chassis.
(**) Coax fed, no balun.

Roy Lewallen, W7EL