I use Eznec to calculate impendences....and also have tried a 1:1 and a 2:1
balun using same 600 ohm feeders...but nothing gives me as close a match as
the 4:1 balun.
----- Original Message -----
From: "Bob McGraw - K4TAX" <RMcGraw@Blomand.net>
To: "Discussion of Ten-Tec Equipment" <tentec@contesting.com>
Sent: Friday, April 11, 2008 5:08 AM
Subject: Re: [TenTec] Ten Tec 238A settings?
> Large variations in tuner settings from one end of 80M to the other end is
> normal. This is about typical of all tuners as the ratio of bandwidth to
> frequency is greater on the lower frequency bands than on higher
> bands/frequencies. I really haven't found a band or frequency or
> reasonable
> antenna below 30 MHz that the 238 tuner would not handle. {I've not tried
> to
> load a coat hanger. hi hi}
>
> I do have a chart based on settings for all bands and several settings vs.
> frequency for each band, and for two different antennas. This makes for
> quick tune-up when changing antennas or bands. I'll be glad to share that
> but it most likely would apply only to my antenna configurations, those
> being a 1/2 wave for 160M fed with balanced line and a 1/2 wave for 75M
> fed
> with coax.
>
> I do not suggest using a 4:1 balun for your configuration but a 1:1
> current
> balun. These are typically less lossy as you are not matching the feed
> line
> Z but the antenna load Z which is likely closer to 35 to 100 ohms or
> less.
> If you have an Antenna Analyzer measure the Z at the balun coax terminal.
> You'll likely find that it is 10 to 25 ohms. This being the case then a
> 1:1
> balun would be more nearly correct as the load with that configuration
> would
> be 40 to 75 ohms. Again, a better match for your coax between the balun
> and
> the tuner.
>
> For whatever reason, using balanced line of some 400 to 600 ohms, hams
> typically believe they should match the feedline Z. In actuality the
> feedline is more correctly reflecting the antenna feed impedance. And is
> some cases, acting like a transmission line transformer.
>
> Here is a case where a 4:1 balun is accurately used. The old fashion
> folded
> dipole made of twin lead. The feed Z of a 1/2 wave folded dipole where the
> top and bottom conductors are of equal size is nominally 300 ohms when
> greater than 1/4 wave above ground. Add an electrical 1/2 wave of
> feedline
> and one has an antenna with a feed impedance at the transmitter of about
> 300
> ohms. Use a 4:1 balun and the transmitter feed is 75 ohms showing a SWR
> of
> 1.5:1 . Drop the antenna down to less than 1/4 wave and the feed Z will
> drop toward 200 ohms which then with a 4:1 balun the feed point for the
> transmitter is now approaching 50 ohms.
>
> 73
> Bob, K4TAX
>
>
> ----- Original Message -----
> From: "Denton" <denton@oregontrail.net>
> To: "Discussion of Ten-Tec Equipment" <tentec@contesting.com>
> Sent: Friday, April 11, 2008 12:28 AM
> Subject: [TenTec] Ten Tec 238A settings?
>
>
>>I have a horizital loop up about 30' that has about 240' of wire in
>>roughly
>> a trapazoid shape....have found that by using a certain length of 600 ohm
>> feeders with a 4 to 1 balun, I have swr below 3.5 to 1 or below on all
>> bands
>> from 80 thru 10 meters, as measured at the balun's input...with the
>> exception of 17 and 60 meters. I am also using about 20 ft of good
>> quality
>> coax to the shack with a TT 238A tuner to take care of the minor swr on
>> the
>> system.
>> I notice that on 75 and 80 meters there is a large variation in tuner
>> settings from one end of the band to the other....is this typical of TT
>> tuners and L networks in general?
>> It appears to be a decent antenna system....if I can hear them, can work
>> them.
>> I have also verified the antenna's effiectiveness by also using it with a
>> link coupled transmatch...by extending the 600 ohm feeders and running a
>> short stubb of 400 ohm line to the shack.
>> TT's L network gives me a much broader matching range, otherwise would
>> just
>> use the balanced transmatch.
>>
>>
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>
>
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