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Hey,
I've got a halfwavelengh dipole hanging on the facade of my balcony, it's two 5 meters wires connected through a 1:1 balun to work in the 20m band. I tried to optimise the length by cutting wire to go to the minimum SWR measured on the nanovna, but to my surprised I ended up with only 4,25m wires, far from the theoretical 5m.
I'm just a newbie so I might have done something wrong but I don-t know what, can anybody give me a hint??
Thanks in advance!!!

First you must include the velocity factor of the wire - typically .95.? That brings the length down to 4.75.? Also, as the wire gets closer to the ground, the ground capacity makes the wire look longer..? 4.25M sounds? a little low.? Are the wires near any metal?

Dave - WB6DHW

Are you using bare copper wire or wire with an insulating cover copper wire like house wiring? Generally when I build an antenna, I start with the following formula taken from the ARRL Handbook: Length (ft) = (492 x 0.95) / f(MHz), where the f(MHz) is the center frequency of the dipole. Once I know the length, I add a couple of additional feet of wire to each end. From there I get the antenna up in the air to take the first measurements to get a reference starting point. Hopefully, the resonant point will be higher than the center frequency you want for the dipole. If so, then you need to bring the antenna down and cut a couple of inches from each end of the wire. Place the antenna back up in the air and recheck your reading, hopefully, you will see the resonant point move towards your desired center frequency, it may not be much of a change, maybe a few MHz. Time to bring the antenna back down and cut some more off the end, and recheck the SWR and resonant point. As you get closer to your center frequency, start cutting a shorter amount of wire off the end. Instead of 2-4 inches, now only cut 1 to 1/2 inch off. If it turns out you went below your center frequency, you will need to start over with new wire or add wire to your existing antenna.

Good Luck

David S

you will see the resonant point move towards your desired center

frequency, it may not be much of a change, maybe a few MHz.

I'm sure you mean a few KHz. Should be clear from context, but just in case
somebody is confused...

Post cited "Hopefully, the resonant point will be higher than the center frequency you want for the dipole". The extra Dipole length (to allow trimming for better Resonance/lower SWR) results in a LOWER resonant frequency than the Target. Shortening Dipole then Raises Frequency towards desired center frequency. Regards, John

On 8/23/20 3:07 PM, davidb1stein@... wrote:
<snip> As you get closer to your center frequency, start cutting a shorter amount of wire off the end. Instead of 2-4 inches, now only cut 1 to 1/2 inch off. If it turns out you went below your center frequency, you will need to start over with new wire or add wire to your existing antenna.

This is why you crumple or fold the wire back, makes it easier to make it longer.

Wire diameter, insulation proximity to other object including wood will all
make a dipole shorter at lowest SWR. Or otherwise said will load the dipole
and make it electrically longer than is is.

Antennas in attic will be shorter than out doors as will low antennas.

Allison
-----------------
No direct email, it goes to bit bucket due address harvesting in groups.IO

Some antenna materials don't fold, bend, or mutilate.
Could cut long, measure the resonant frequency, then reduce in length
by a ratio proportional to FreqResonant/FreqTarget.

Jerry, KE7ER

When I trim an antenna I first measure the resonant frequency.? Then compute the ratio of that to the desired frequency.? Convert that to wavelength, divide by 4 (each side of a dipole is 1/4 wave) and that will give me the desired change.? To avoid overshooting, I will only change half that amount and then repeat the process.? Of course, in the stuck-in-the-mud USA, convert meters to inches.
You don't have to measure what you have.? The resonant frequency tells you that.
Bob K6DDX

Thanks for your answer Dave, in fact there is wire everywhere I guess, the fences of the balcony and I guees many other in the structure of the building.

Thanks David, that's basically what I did, except that I was only checing the minimum SWR, How should I do to find the resonance freq with nanovna??

Thanks Allison!!

Sorry guys, must have a brain freeze when I wrote part of my response, you are correct the longer the wire, the lower the frequency, so you would be starting off lower than where you want to be and work your way up to the frequency you want to operate at.

--
David S

You can measure resonance with the nano by observing the phase.? When the phase switches from 180 degrees to -180 degrees you have resonance.? But minimum SWR is usually a good enough indicator if it's fairly low, say less than 2:1.
Bob K6DDX

It's a bit more complicated than that.

When the resistance at resonance is less than the 50 ohm impedance
of the VNA, the reflection is 180 degrees out of phase with the incident signal.
This would be the case for a yagi or quarter wave whip.

But when the resistance at resonance is greater than the 50 ohm impedance
of the VNA, the reflection is in phase with the incident signal.
This would be the case for a dipole.

I use the smith chart display, the antenna is purely resistive where the plot
crosses the horizontal axis in the center of the screen. It crosses to the left
of center if the antenna is less than 50 ohms, to the right of center if the antenna
is greater than 50 ohms. Drag a marker around the plot using the touchscreen,
text on the screen will tell you the frequency at that position.

However, I have built antennas where the smith chart plot circles the 50 ohm
origin, so I have a pure resistance at two positions. One greater than 50 ohms
and one less than 50 ohms. Not quite sure exactly what that means.


When trimming a yagi or quarter wave whip for best SWR, an in tune antenna would
be too low of impedance. So you wind up with a trim that has extra reactance to
best match 50 ohms. Usually close enough, but the antenna is not quite in tune.

I believe that when trimming a dipole for best SWR, you can wind up with an
antenna in tune. The resistance will be a bit higher than 50 ohms, but any
other trim will be even higher.

Take the above with a very large grain of salt.
I've only looked at a handful of antennas.

Jerry, KE7ER

1) Yes, measure the frequency where the phase shifts from +180 to -180
degrees.

2) Using the definition of resonance: Measure the frequency where the
complex portion of the impedance goes to zero.

In reality, both methods are measuring the same parameter, just presented
in different formats.

If you view this on the Smith Chart, either measurement would yield the
frequency where the measurement goes between the top and the bottom of the
chart across the single horizontal line on the chart. That horizontal
represents resistance only, no reactance.

Dave - W?LEV

On Mon, Aug 24, 2020 at 4:37 PM Bob Albert via groups.io <bob91343=
[email protected]> wrote:

You can measure resonance with the nano by observing the phase. When the
phase switches from 180 degrees to -180 degrees you have resonance. But
minimum SWR is usually a good enough indicator if it's fairly low, say less
than 2:1.
Bob K6DDX
On Monday, August 24, 2020, 02:58:45 AM PDT, <ea3iji@...> wrote:

Thanks David, that's basically what I did, except that I was only checing
the minimum SWR, How should I do to find the resonance freq with nanovna??

--
*Dave - W?LEV*
*Just Let Darwin Work*

As Miro has pointed out, tuning an antenna for resonance by looking for a pure resistance
in the nanovna display must be done when measuring at the antenna feedpoint.

This can be done by attaching your already calibrated VNA directly to the feedpoint.

Or you can perform the calibration procedure with the nanovna
in the shack, the Open-Short-Load devices at the antenna end of the coax.
This will allow the VNA to calibrate out the effects of the coax.

Once the antenna is tuned to be purely resistive, just hook up the coax and
use your antenna tuner in the shack to make the antenna system look like 50 ohms
to your transmitter.
There will be some reflections in the coax between antenna and antenna tuner
if the antenna is not exactly 50 ohms, these reflections will burn up some of the
transmitter's power as heat due to attenuation (mostly resistive losses in the
not-quite-ideal coax), but below 30 mhz with 100 feet of RG8 these losses
should be insignificant.
Some in the forum may disagree, feeling you should climb up on the roof and
twiddle knobs to tune your antenna whenever you move to the far end of the band.
But Maxwell disagrees, it's the one point that he keeps hammering away at
in Reflections III, and in his half dozen articles in QST around 1973.
Other than the losses in the non-ideal coax incurred by a few round trips that a
fraction of the transmitted power takes due to reflections, all of the transmitter's power
eventually goes out the antenna.

Antennas get really complicated when you dig deep into the details.
And are fodder for all sorts of arguments.

Jerry, KE7ER

Point 1) is wrong (if resistance is greater than 50 ohms),
Point 2) is correct.

Use your nanovna to measure the phase of S11 for a resistor
that has a value between perhaps 60 and 100 ohms.
Keep the frequency to something reasonable, say below 100 mhz,
so you don't have to worry about resistor lead length.
Phase is zero.

To do this, select a trace, select channel 0, then select a format of "phase".

Only if the resistor is below 50 ohms does the phase flip to 180 degrees.

In both cases, the complex portion is zero, and we are measuring a pure resistance.

Jerry, KE7ER

Arguments and snake oil regarding antennas and transmission lines should
not prevail when the hard, cold, correct, engineering practices and hard
physics of the situations are considered. These considerations should
always prevail over guesses, maybes, 'it works', but...., too much
math,......., I can't find how to use this thing,........snake oil to
explain away improper measurements.........

The NANOVNA's are great instruments for answering all these questions in a
rigorous manner true to engineering and physics. Frankly, they're the best
thing to hit amateur radio and other RF disciplines since sliced bread.
Take the time and expend the effort to learn how to use them. You'll never
be sorry. Those of us in this group will help you learn. Experience is
the best teacher. Measuring antenna resonances is a good place to start!
And, the cost is about that of a hard-bound ARRL Handbook which does not,
in itself, contain as much information as you will learn in using the
NANOVNA's.

Just another personal pitch for a job well done by its designers and
coders!!! THANK YOU!!! The closest competition to these units comes in
around $380 and is nowhere near as capable as these units.

Dave - W?LEV

On Mon, Aug 24, 2020 at 6:22 PM Jerry Gaffke via groups.io <jgaffke=
[email protected]> wrote:

As Miro has pointed out, tuning an antenna for resonance by looking for a
pure resistance
in the nanovna display must be done when measuring at the antenna
feedpoint.

This can be done by attaching your already calibrated VNA directly to the
feedpoint.

Or you can perform the calibration procedure with the nanovna
in the shack, the Open-Short-Load devices at the antenna end of the coax.
This will allow the VNA to calibrate out the effects of the coax.

Once the antenna is tuned to be purely resistive, just hook up the coax
and
use your antenna tuner in the shack to make the antenna system look like
50 ohms
to your transmitter.
There will be some reflections in the coax between antenna and antenna
tuner
if the antenna is not exactly 50 ohms, these reflections will burn up some
of the
transmitter's power as heat due to attenuation (mostly resistive losses in
the
not-quite-ideal coax), but below 30 mhz with 100 feet of RG8 these losses
should be insignificant.
Some in the forum may disagree, feeling you should climb up on the roof
and
twiddle knobs to tune your antenna whenever you move to the far end of the
band.
But Maxwell disagrees, it's the one point that he keeps hammering away at
in Reflections III, and in his half dozen articles in QST around 1973.
Other than the losses in the non-ideal coax incurred by a few round trips
that a
fraction of the transmitted power takes due to reflections, all of the
transmitter's power
eventually goes out the antenna.

Antennas get really complicated when you dig deep into the details.
And are fodder for all sorts of arguments.

Jerry, KE7ER



On Mon, Aug 24, 2020 at 10:37 AM, Jerry Gaffke wrote:

It's a bit more complicated than that.

When the resistance at resonance is less than the 50 ohm impedance
of the VNA, the reflection is 180 degrees out of phase with the incident
signal.
This would be the case for a yagi or quarter wave whip.

But when the resistance at resonance is greater than the 50 ohm impedance
of the VNA, the reflection is in phase with the incident signal.
This would be the case for a dipole.

I use the smith chart display, the antenna is purely resistive where the

plot

crosses the horizontal axis in the center of the screen. It crosses to

the

left
of center if the antenna is less than 50 ohms, to the right of center if

the

antenna
is greater than 50 ohms. Drag a marker around the plot using the

touchscreen,

text on the screen will tell you the frequency at that position.

However, I have built antennas where the smith chart plot circles the 50

ohm

origin, so I have a pure resistance at two positions. One greater than

50 ohms

and one less than 50 ohms. Not quite sure exactly what that means.

When trimming a yagi or quarter wave whip for best SWR, an in tune

antenna

would
be too low of impedance. So you wind up with a trim that has extra

reactance

to
best match 50 ohms. Usually close enough, but the antenna is not quite in
tune.

I believe that when trimming a dipole for best SWR, you can wind up with

an

antenna in tune. The resistance will be a bit higher than 50 ohms, but

any

other trim will be even higher.

Take the above with a very large grain of salt.
I've only looked at a handful of antennas.

Jerry, KE7ER

--
*Dave - W?LEV*
*Just Let Darwin Work*

Hi ea3jii and others,

Antennas and particularly dipoles on our HF bands are a sum of compromises due to influence of the construction and environment.

A pure dipole should have an impedance of Z=73.1+j0, but on HF all the above parameters tend to have around 50 Ohm in practice... The fun starts with the 95% "shorting" factor, the ratio wave length/wire diameter, wire type (up to 5% shorter with PVC unsolated wire), antenna heigth above ground, antenna shape, etc, etc...

Understanding your environment, (fences, balcony, etc...) it's normal that you have a much shorter length than calculated, and in a completely unpredicted way...

I tune my antenna to resonance when j=0 and "accomodate" the factors on which i have influence (length, shape,...) to bring this resonance and impedance on the frequency i want.

But as Jeff KE7ER mentionend, you have to take some precautions when measuring.
Before the VNAs, we used feeders with halfwave length (or multiples) to be sure that what we measured on the end of the feeder was (approx.) the same at antenna feedpoint. With a VNA, you calibrate it with your feedline connected and you're good (almost).
Follow Jeffs KE7ER advices and you will be good.

But in your situation, i predict that you will have a poor performer, and maybe have to change to another type, perhaps a vertical and EFHW.
I've worked more than 300 DXCC with a vertical antenna that had only one "radial" per band with which i could adapt impedance and resoncance by pruning only the radial length and angle. It can considered like a vertical bended dipole.

Make your own experience ! You have all the tool and infos needed.. and enjoy !

73 Patrick, TK5EP