"Proximity to ground and other structures will detune your antenna -
making a
calculated length actually resonant at a higher frequency. ..."
For antennas at heights between 1/4 wavelength and 1/2 wavelenght, the
resonant frequency goes up with increasing height. For antennas at
heights between 1/2 and 3/4 wavelength, the resonant frequency goes down
with increasing height. This pattern repeats every half wavelenght in
height, decreasing in amplitude as a damped sinusoid.
For antennas less than 1/4 wave high (which this one is), the answer is
more complicated and depends on your ground quality. For excellent
ground the frequency can go down with increasing height, and for average
or poor ground, the frequency goes up with increasing height.
These variations are not very large, maybe 30 KHz at 160 meters,
certainly not anything close to explaining the 470 KHz difference Jon
experienced. (2.1 MHz Jon measured, and 1.63 MHz I simulated).
Jerry, K4SAV
David J. Ring, Jr. wrote:
>Jon,
>
>Two things explain your problem - and they're both "to do" because of
>"wavelength".
>
>First your 160m OCF is very close to ground in terms of wavelenth.
>
>
>1.8 MHz is 166.67 meters
>2.0 MHz is 150 meters.
>
>Difference: 16.67 meters or about .
>
>3.5 MHz is 85.71 meters
>4.0 MHz is 75 meters or about 18 feet difference.
>
>Difference: 10.71 meters or about 11 feet difference.
>
>Proximity to ground and other structures will detune your antenna - making a
>calculated length actually resonant at a higher frequency. We usually add
>5% to the calculated length of a diple (already factored into the 468/f(MHz)
>formula. Closeness to ground and the proximity effect is proportional to
>the wavelenth of the frequency under consideration. More effect at greater
>wavelengths. Thus 160 m suffers most of all from this as 50 feet is but 15%
>or so of a wavelength at 1.8 MHz. (I've only used 39 inches equals one
>meter in the above calculations, so my figures aren't exact. I also used
>300/f(MHz) to calculate wavelength.)
>
>In all cases it is "better" to have an antenna slightly longer than slightly
>shorter. You can take away but it is structuraly weaker to "add on".
>
>You might consider the Inverted L antenna for 160 meters if you have the
>room - it gives superior performance on both close in and distant
>propagation. If you're just interested in DX and don't mind if your high
>angle radiation is surpressed, go for an Tee antenna. You'll need twice the
>top section however for the Tee antenna. (These are wonderful antennas.)
>
>W1BB - Stew Perry did many experiments with top band (160 meter) antennas
>and he really thought the Inverted L with a 1/4 wave resonant top section
>(around 135 feet) on top and "whatever vertical section was available" was
>the "best" antenna for many people. This antenna worked well for local and
>for DX but it had to be matched with a capacitor as it would be much longer
>than the standard 1/4 wave vertical (or Inverted L).
>
>One trick that I've used to benefit when I couldn't put up such an antenna
>(extended 1/4 wave inverted L) and could only fit a 1/4 wave length of wire,
>was to use "twin lead" or TV line. You feed one wire at the beginning, then
>short both wires at the far end, and the remaining wire at the beginning
>gets connected to ground. This arrangement doubles the radiation resistance
>and consequently halves the ground losses of the antenna.
>
>In this design, you take a 135 foot section of twin lead (75 ohm or 300 ohm)
>and short the "far end" by stripping the insulation, and tying the wires
>together and soldering them. Using twin lead feedline has a problem.
>Supporting 135 feet of this stuff by tying an insulator on the end of the
>"shorted and soldered" end (by making the stripped ends of sufficient length
>to allow an insulator to be placed there) is a problem because the wire
>frequently (also known as "always") breaks at the end. To eliminate this,
>you can take two pieces of plexiglass of a size about 6 by 4 inches, score
>the area so that the twin lead will "fit" into the groove, and then with
>small brass or stainless bolts and nuts, secure the sandwitch. I seal the
>end of the twin lead with "Liquid Tape" after wrapping it with a good grade
>of plastic electricians tape. DO NOT OMIT this part, and do it to ALL
>outside antenna connections. It is a valuable "trade secret!
>
>To do this easily, get some round (not lockwashers) and large pancake
>washers - you'll need about four or five depending on the thickness of the
>twin lead per bolt. Use these inside to allow the twin lead to be held
>comfortably.
>
>I've also scored the plexiglass at right angles to the twin lead - it will
>behave like the serated jaws of a Vice-Grip (tm) pliers and hold the twin
>line sufficently.
>
>What the twin lead does is to double the impedance of the antenna. If the
>measured antenna has a typical value of 36 ohms (which is standard for a 1/4
>wave vertical wire), then having the two wires will double it to around 70
>ohms. Also the ground losses as they are proportional to their fraction of
>the radiational resistance of the antenna will go down by a factor of two.
>
>Some people have used this 160 meter 1/4 wave antenna and put a series trap
>(resonant at 3.7 MHz) a distance of about 55 feet from the feed point on 80
>meters, the antenna appears to have a short at 55 feet, and thus appears to
>be a 1/4 wave antenna on that band. However, using this arrangement you
>cannot use the 55 foot long antenna on 30 meters. With a wire to the coil
>of the relay this antenna will then work on 160/80 and the 80 meter antenna
>will perform on the 3rd harmonic or about 10.5 MHz. (Just like a 7 MHz
>dipole works on 21 MHz).
>
>I hope this information is helpful to you.
>
>73
>
>David N1EA
>
>_______________________________________________
>
>
>
>_______________________________________________
>TowerTalk mailing list
>TowerTalk@contesting.com
>http://lists.contesting.com/mailman/listinfo/towertalk
>
>
>
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|