When loading a high impedance line with a low impedance antenna (a
nearly half wave antenna, like yours on 40m is low impedance) a quarter
wave of line gives you a very high impedance. That antenna has a
resistive component about 60 ohms and some reactance. With a 600 ohm
characteristic impedance the impedance a quarter wave away is 600 * 600
/ 60 or 6000 ohms. And a KW at 6000 ohms means e = sqrt( 1000 * 6000) =
2500 volts. If the feed line impedance was 300 ohms, the impedance a
quarter wave from the antenna would be 1500 ohms and the voltage for 1
KW would be only 1250 volts.
Your spreaders are two wide. They are abouto 4.5" long for 600 ohms and
300 ohms would require just under a half inch spacing.
65 feet of feed line comes to about a half wave at 40 meters and that
means the tuner sees practically the antenna impedance which is more
like 60 ohms.
There is more than one way to cope with the high impedance, adding feed
line is one sure way. Lowering the impedance of the feed line is
another. Now that your feed line is about a half wave on 40 meters, its
a multiple of that half wave on 20, 15, and 10 meters always reflecting
the dipole impedance on those bands to the tuner and the dipole
impedance on those bands is always high, but not like what the tuner saw
with they near quarter wave feed line.
73, Jerry, K0CQ
On 2/16/2011 8:13 AM, tom stewart wrote:
OK,
It is OK now but WHY? Everything I learned,studied and observed on line and
in commercial antennas this size says that 45-50 should have been the
correct length. Now I am at between 60-65 feet. Is it my surroundings? The
actual attributes of my 14 gauge THHN wire? Are my spreaders not what they
should be?
I will let this thread die now- thank you all very much for your generous
efforts on my behalf.
Tom Stewart N7ESE TenTec and CW Love ham radio
On Tue, Feb 15, 2011 at 4:54 PM, Rick - NJ0IP / DJ0IP<Rick@dj0ip.de> wrote:
We already did that yesterday Stuart.
He added 13 feet and now it is working all bands.
We're just fine tuning today.
73
Rick
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