Bob,
I agree with you but let me play devil's advocate...
There are those who say a center-fed dipole, say for 80m, fed with openwire,
with have a high impedance on its even harmonic - a couple thousand Ohms.
Doesn't this result in excessive power loss in the matchbox? The OCF on the
other hand will have impedances between about 50 and 800 ohms so with a 4:1
balun, the SWR still never exceeds 4:1, a much friendlier range for
matchboxes to match.
73
Rick, DJ0IP
-----Original Message-----
From: TenTec [mailto:tentec-bounces@contesting.com] On Behalf Of Robert
Mcgraw
Sent: Tuesday, October 16, 2012 8:28 PM
To: k9yc@arrl.net; Discussion of Ten-Tec Equipment
Subject: Re: [TenTec] Model 238C Tuner Questions
For the very reason Jim states, I do not like antennas fed from anywhere but
the center. Either use a balanced feed system or use a good 1:1 current
balun or choke of suitable RF power rating {if in doubt go for a higher
power rating} at the feed point when using a coax feed system.
I know that many say OCF antennas work and work well. However there is most
always feed line radiation and that means feed line reception too.
With the feed line being basically vertical, we find that vertical antennas
and vertical feed lines are very prone to receiving local noise.
A balanced antenna feed has and offers a high degree of common mode signal
rejection. This common mode rejection makes for several dB of reduced
noise.
Balanced feed systems are really quite easy to install and use and are
proven to be the highest efficient means of transferring RF. I know what
you've been told by others about balanced feed systems. From my experience,
what I've been told by others is largely at least 90% wrong.
The other 10% is questionable. I run a balanced feed from the top of the
tower, down the tower on suitable stand-offs, through the eve vent, across
the attic, and it drops down through the ceiling to the tuner.
Nope, no coax or balun to get to the outside either. That defeats the
general purpose.
Yes the procedure in the manual works well. Start with the lowest frequency
bands first, make a chart and move to the higher frequency bands. As one
gets to the higher frequencies, the tuning controls are more critical but
the actual tuning range is much wider than many other types of tuners.
73
Bob, K4TAX
> On 10/16/2012 4:19 AM, Richards wrote:
>> Does anybody have a simple description of the procedure one should
>> use to quickly find the sweet spot?
>
> There's a procedure in the manual, which is not in front of me at this
> time, but from memory it goes something like this:
>
> Using small settings for L and C, switch the center knob from its off
> setting to high and to low, observing which seems to reduce SWR and
> use that setting as a starting point. Now, adjust C and L for reduced SWR.
> When you switch the center knob you are adding capacitance, so when
> you increase the C with that switch you should retune the capacitor to
> minimum and continue.
>
> The trickiest matches to find are those with small values of C -- it
> can take a while to figure out whether the switch should be left or right.
>
> In general, high values of L and C are needed on the lower bands, very
> small ones on the higher bands.
>
> The Ten Tec tuners DO tend to stay matched over a wider range than
> most tuners, but the antenna you describe is not well behaved, and a
> poor choice if you have local noise because it is inherently severely
> imbalanced. That imbalance puts considerable common mode current on
> the feedline, which really needs a choke, but the severe imbalance
> puts so much common mode voltage across the choke that VERY high
> values of choking Z are required if the choke is to handle high power.
> OCF dipoles are notorious for frying chokes.
>
> 73, Jim K9YC
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