Posted by
John Young-10 on
Mar 14, 2006; 9:53pm
URL: http://elecraft.85.s1.nabble.com/Strange-KXAT1-SWR-goes-up-but-so-does-forward-power-Baluns-tp387591p387592.html
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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