Most issues of power loss in a tuner is the result of IR loss being that
the network is operating with a very low Z load. If my antenna is a 1/2
wave, resonant and fed in the center, the typical feed Z is about 30 to
50 ohms. With a balanced feed, dump that into a 4:1 balun and the balun
input is now about 10 to 12 ohms. Lots of current flowing at legal limit
power. Hence a good deal of IR loss in the components or one must have
very large components.
The only issues I've had with very high antenna Z would be more likely of
voltage breakdown of components, flash over and such. I've smoked a few
baluns which were on the output of a tuner. In all cases, the antenna
load was quite high and all suffered voltage breakdown, subsequent smoke
and fire inside the box. To that end, I now operate my tuner with a
balun on the input thus it, the balun, is always working at near 50 ohms
non-reactive. Of course that requires a tuner other than the 230. Your
charts on the various tuners is a wealth of information.
The old Johnson Matchbox is the "standard" to which most tuner designs
would like to attain. In my case, the Johnson 250 Matchbox will put the
135 ft center balanced fed wire on any band, 80M - 10M and with excellent
match and very low loss.
73
Bob, K4TAX
> 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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>>
>
>
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