Strange: KXAT1: SWR goes up, but so does forward power?? +Baluns

[Martin Gillen-2]

Hi,

I recently built a W1CG low power balun (Guanella 4:1 two-core design).

I'm pretty new to amateur radio and my on-air results were pretty
confusing to me
so I was hoping someone could help me understand what is or may be going on
here:

For these tests I was using an inverted vee dipole, each leg 32ft
(64ft total length),
fed in the center with 32ft radio shack 300 ohm twinlead, held aloft
by a 20ft fiberglass
fishing pole, with the ends held 6ft off the ground by fiberglass
driveway markers.

Rig was a KX1 with KXAT1 ATU.  Forward power and SWR readings taken from
the KXAT1 (ie. no external metering).

I started by connecting the twinlead directly to the KX1 using a binding post to
BNC adapter.  I was able to tune the radio on 40m, 30m and 20m to under 1.5:1
SWR, often 1.0:1.  Power out read as:

1.5W on 40m
3W on 30m
2W on 20m

I then replaced the binding post to BNC adapter with the 4:1 balun.  Now the SWR
increased on all 3 bands:

1.8:1 on 40m.
3.2:1 on 30m.
4.2:1 on 20m.

On 20m and 30m there was a noticeable attenuation of signals but 40m
sounded fine.

So... so far it looks like the antenna simply didn't need the
impedance transformation,
and has degraded all 3 bands.

I gave up on 20m and 30m with the balun due to the high SWR.

However the odd thing I noticed was that the forward power reported by the KXAT1
on 40m had increased to 3W!

So... I established contact with a station and asked him how my signal
sounded with
and without the balun.

Here is the summary:

Without Balun:

KXAT1 SWR 1.0:1
KXAT1 Forward Power 1.5W
Signal report: 579

With Balun:

KXAT1 SWR 1.8:1
KXAT1 Forward Power 3W
Signal report: 599+

I switched the balun in and out a few times to accound for fading.

So I am left confused.  Why did the SWR go up if the forward power
also went up?

My working theory is that the antenna fairly well matched already on all 3 bands
but unbalanced.  Adding the 4:1 balun had two effects:  (1) Impedance
transformation
(2) Balance the transmission line (& dipole).  The impedance
transformation wasn't
actually required and actually placed the impedance outside the KXAT1 matching
range on 20m/30m, and degraded the SWR on 40m, but the balancing of the
transmission line meant more energy radiates from the dipole elements and not
from the transmission line.

So the data seems to be telling me that this antenna could really use
a 1:1 balun.

What am I missing??

:)

73
Martin.
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[dave-281]

Martin,

Not sure if I can explain this adequately or not, but here goes.

When you have a mismatch at the antenna - and you always will with 300 ohm twin lead -
there is energy reflected from the ant/feedline junction.  That energy is returned along
the feedline to the transmitter/tuner.  It must then be re-reflected back toward the
antenna.

When properly tuned, the tuner's impedance is such that the re-reflection occurs on the
ant side of the tuner and no reflected energy is observed between the tuner and transmitter.

But, when the tuner is mis-tuned, the energy goes through the tuner on to the TX, where
part of it is re-reflected back toward the ant.  When this happens, both the forward and
reflected power will indicate as increasing, because they are.

By tuning properly the reflected energy is never admitted between the tuner and the TX and
is completely re-reflected at the ant side of the tuner, so the SWR meter indicates zero
reflected energy.

This means that the optimum point of tuning is when both the forward and reflected power
are at minimum.

hth

73 de dave
ab9ca






Martin Gillen wrote:

>
> So I am left confused.  Why did the SWR go up if the forward power
> also went up?
>

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[John Young-10]

Martin
     Some basic things that can never be repeated too often.

    Any watt meter will only read correctly when connected to the load
it is designed for.  The power
readings of forward power must be made into a 50 ohm resistive load.  
Your 4.2:1 SWR could be 210 ohm
(4.2 x 50) or 12 ohms (50/4.2).

The dipole will show something in the ballpark of 50 ohms.  When
connected to 32 ft of 300 ohm feed line,
your radio and tuner will see something close to 1500-2000 ohms which is
out of the range of the tuner.

A 40 meter dipole will not tune well on 30 or 20.

Strange as it sounds using an antenna that is not resonant on any of the
bands can work better than one that
is resonant on one band.  Try 25 ft of wire going up the pole and 16 ft
of wire laying on the ground.  connect both
directly to the binding post to BNC adapter and let us know how it works.

Don't worry about sounding like a beginner - I've been a beginner for
over 40 years and still learning.

73
John
WA8KNE


Martin Gillen wrote:

> Hi,
>
> I recently built a W1CG low power balun (Guanella 4:1 two-core design).
>
> I'm pretty new to amateur radio and my on-air results were pretty
> confusing to me
> so I was hoping someone could help me understand what is or may be going on
> here:
>
> For these tests I was using an inverted vee dipole, each leg 32ft
> (64ft total length),
> fed in the center with 32ft radio shack 300 ohm twinlead, held aloft
> by a 20ft fiberglass
> fishing pole, with the ends held 6ft off the ground by fiberglass
> driveway markers.
>
> Rig was a KX1 with KXAT1 ATU.  Forward power and SWR readings taken from
> the KXAT1 (ie. no external metering).
>
> I started by connecting the twinlead directly to the KX1 using a binding post to
> BNC adapter.  I was able to tune the radio on 40m, 30m and 20m to under 1.5:1
> SWR, often 1.0:1.  Power out read as:
>
> 1.5W on 40m
> 3W on 30m
> 2W on 20m
>
> I then replaced the binding post to BNC adapter with the 4:1 balun.  Now the SWR
> increased on all 3 bands:
>
> 1.8:1 on 40m.
> 3.2:1 on 30m.
> 4.2:1 on 20m.
>
> On 20m and 30m there was a noticeable attenuation of signals but 40m
> sounded fine.
>
> So... so far it looks like the antenna simply didn't need the
> impedance transformation,
> and has degraded all 3 bands.
>
> I gave up on 20m and 30m with the balun due to the high SWR.
>
> However the odd thing I noticed was that the forward power reported by the KXAT1
> on 40m had increased to 3W!
>
> So... I established contact with a station and asked him how my signal
> sounded with
> and without the balun.
>
> Here is the summary:
>
> Without Balun:
>
> KXAT1 SWR 1.0:1
> KXAT1 Forward Power 1.5W
> Signal report: 579
>
> With Balun:
>
> KXAT1 SWR 1.8:1
> KXAT1 Forward Power 3W
> Signal report: 599+
>
> I switched the balun in and out a few times to accound for fading.
>
> So I am left confused.  Why did the SWR go up if the forward power
> also went up?
>
> My working theory is that the antenna fairly well matched already on all 3 bands
> but unbalanced.  Adding the 4:1 balun had two effects:  (1) Impedance
> transformation
> (2) Balance the transmission line (& dipole).  The impedance
> transformation wasn't
> actually required and actually placed the impedance outside the KXAT1 matching
> range on 20m/30m, and degraded the SWR on 40m, but the balancing of the
> transmission line meant more energy radiates from the dipole elements and not
> from the transmission line.
>
> So the data seems to be telling me that this antenna could really use
> a 1:1 balun.
>
> What am I missing??
>
> :)
>
> 73
> Martin.
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>  

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[Thom LaCosta]

On Tue, 14 Mar 2006, John Young wrote:

>
> Don't worry about sounding like a beginner - I've been a beginner for over 40
> years and still learning.

Great Outlook!

73,Thom-k3hrn
www.zerobeat.net Home of QRP Web Ring, Drakelist home page,Drake Web Ring,
QRP IRC channel, Drake IRC Channel, Elecraft Owners Database
www.tlchost.net/hosting/  ***  Web Hosting as low as 3.49/month
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[Nick Waterman]

Thom R LaCosta wrote:
>> Don't worry about sounding like a beginner - I've been a beginner for
>> over 40 years and still learning.
>
> Great Outlook!

Hey, we call ourselves AMATEURS for a reason, right?   :-)

--
"Nosey" Nick Waterman, G7RZQ, K2 #5209.
use Std::Disclaimer;    [hidden email]
Why did the tachyon cross the road? Because it was on the other side.
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