First homebrew antenna and a question
 

On Fri, 29 Sep 2006 08:10:01 -0400, jawod wrote:

jawod wrote:
I spent the day lowering the folded dipole that I BOUGHT. I was unable
until recently to find how bad the SWR was on most bands. The only
reasonable SWR was on 17 meters.

So, I replaced it with a homemade G5RV, measured appropiately for each
leg of the dipole (14AWG) and for the 300 ohm window line to R/G8U.

I now find reasonable SWR on most bands (WARC, not so much). Here's the
deal: min SWR comes in right at 3.5 MHz, 6.75, 13.43, 18.07 and 29.06.
It appears that I made the classic newbie mistake...antenna too short.

Now, since I am using an ATU, I think this should be close enough for
acceptable efficiency (at least for 80, 40, 20, 17 and 10 meters).

What do you think?

John
AB8O

PS, When I get the time, I'll try EZNEC but for now, I just want to work
what I hear for a change (!)
Update:

Well, yes, the antenna was too long (not too short). I decided to lower
the antenna and shorten each leg by 18 inches. Now it's back up about
45 feet, sloping to about 25 feet. (No change from before.)

Now, SWR dips are at 3.610, 6.710 and 13.580 MHz.

These are not what was expected. Oddly, on 40M SWR dip went further
away from the band edge (as if dipole was lengthened).
I did not change the 300 ohm window line (at 31 feet).

From what I've read here and elsewhere, I should note 80 and 20M
performance as primary considerations.

I'm a little nervous about shortening it further.


I have explained to you several times that the length of the radiator
and the open wire section act together to influence the impedance
presented at the coax to open wire junction.

I asked if you had measured both elements and if you had considered
the velocity factor (for your actual line) in those measurements, but
you did not reply, and you have not reported the length of the open
wire line. The surest way is to measure the velocity factor, calculate
the correct length and accurately cut to that length.

The common G5RV has dipole's second series resonance at 14.2 and the
open wire line an electrical half wave at 14.2, which will present a
resistive load of somewhere 80 to 90 ohms at the coax to openwire
junction, and you will observe a VSWR minimum on the coax at that
frequency (even at the tx end of the coax line). (Of course if you do
something silly like use a 50 ohm VSWR meter in a 75 ohms coax line,
you aren't making valid VSWR observations.)

Remember the rule: measure twice, cut once.

It can't be all that hard, can it.

Owen

PS: I noted your throw away comment that time is of the essence, the
wet season is coming. Have you noted the recent comments on
performance of wet ladder line. Depending on the construction, your
300 ohm line might be subject to similar degradation, and your
"optimised" configuration might not be optimised when the feedline is
wet.
--

First homebrew antenna and a question
 

I have explained to you several times that the length of the radiator
and the open wire section act together to influence the impedance
presented at the coax to open wire junction.

I asked if you had measured both elements and if you had considered
the velocity factor (for your actual line) in those measurements, but
you did not reply,





It can't be all that hard, can it.



PS: I noted your throw away comment that time is of the essence,

Owen,

I appreciate your advice.

Just glad I'm not your neighbor.
I'm certainly not your errant student.

There are some elmers out there and then there are responses as above.

If you feel it is beneath you to respond, then by all means don't waste
your time responding. This is all too familiar.

For the record, I did not have deviating from G5RV specs as part of my
"design" and, as a first attempt, I wanted to avoid higher order issues
such as the arcane bickering about water on a transmission line.

Frankly, from a newbie perspective, so much controversy over such issues
calls into question any science that is behind so many assertions and
counter-assertions. If so much depends upon water on the line, then how
important can velocity factor be? This is said only partially in jest.

In my mind MIS-application of science is far worse than its non-application.

I think a good engineer would agree with me. Would you?

John
AB8O
not an engineer

PS
Cecil, your advice is more on the mark for me and I appreciate it. I'll
work on velocity factor and EZNEC for the next project...if time allows.
I'm proud to say that this is my hobby but it's not my career.

First homebrew antenna and a question
 

jawod wrote:
For the record, I did not have deviating from G5RV specs as part of my
"design" ...

If you used 300 ohm ladder-line, like The Wireman's
#561 or #562, you did indeed deviate from G5RV specs
by making the ladder-line length equal to 31 feet.
For that particular transmission line, 1/2WL on 20m
is around 28 feet.
--
73, Cecil http://www.w5dxp.com

First homebrew antenna and a question
 

Cecil Moore wrote:
jawod wrote:

For the record, I did not have deviating from G5RV specs as part of my
"design" ...


If you used 300 ohm ladder-line, like The Wireman's
#561 or #562, you did indeed deviate from G5RV specs
by making the ladder-line length equal to 31 feet.
For that particular transmission line, 1/2WL on 20m
is around 28 feet.
Yes, and thanks
I actually emailed the supplier as there are no markings at all
indicating type on the ladder-line cable, but based upon description, I
have the 300 ohm line. Per your advice, I'll try shortening the
transmission line. Yours (and even Owen's) advice are always appreciated.

I simply won't be belittled or badgered, well, for ANY reason.

I really do listen to you guys, but practicality and time constraints
rule the day.

73,
John
AB8O

First homebrew antenna and a question
 

On Sat, 30 Sep 2006 01:16:44 GMT, Cecil Moore
wrote:

jawod wrote:
For the record, I did not have deviating from G5RV specs as part of my
"design" ...

If you used 300 ohm ladder-line, like The Wireman's
#561 or #562, you did indeed deviate from G5RV specs
by making the ladder-line length equal to 31 feet.
For that particular transmission line, 1/2WL on 20m
is around 28 feet.

Wireman's data for #561 and #562 shows vf=0.91, and for #563, vf=0.66.

I have found wide variation in line construction and hence vf of
nominally 300 ohm line, and suggest that it is worth measuring.

This information is not new, G5RV wrote "If it is desired to use 300
ohm ribbon type feeder for this section, it is strongly recommended
that the type with "windows" (ladder line) be used because of its much
lower loss than that with solid insulation throughout its length, and
its relative freedom from the "detuning" effect caused by rain or
snow. If this type of feeder is used for the matching section,
allowance must be made for its velocity factor in calculating the
mechanical length required to resonate as a half-wave section
electrically at 14.150 MHz. Since the velocity factor of standard 300
ohm ribbon feeder is 0.82, the mechanical length should be 8.5m (28
ft). However, if 300 ohm ribbon with "windows" is used, its velocity
factor will be almost that of open-wire feeder, say 0.90, so its
mechanical length should be 9.3m (30.6 ft)."

Talking about the physical length of the so-called matching section
without knowing the velocity factor of the actual line is incomplete,
it is the electrical length that matters.

Owen
--

First homebrew antenna and a question
 

Owen Duffy wrote:
Wireman's data for #561 and #562 shows vf=0.91, and for #563, vf=0.66.

I have found wide variation in line construction and hence vf of
nominally 300 ohm line, and suggest that it is worth measuring.

I use #562 on my 20m rotatable dipole. Its VF measures
very close to 0.8. I needed the accuracy because I
vary the length of the feedline in order to resonate
the system on 20m-6m.
--
73, Cecil http://www.w5dxp.com

First homebrew antenna and a question
 

jawod wrote:
I simply won't be belittled or badgered, well, for ANY reason.

In Transactional Analysis, it's known as the
parent to child communications path. :-)
(We should all use the adult to adult path.)
--
73, Cecil http://www.w5dxp.com

First homebrew antenna and a question
 

Talking about the physical length of the so-called matching section
without knowing the velocity factor of the actual line is incomplete,
it is the electrical length that matters.

Owen
--
Fair enough, Owen. Now, was THAT so hard?