rectangle antenna for 10m

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rectangle antenna for 10m Empty Re: rectangle antenna for 10m

Post by Admin on Tue Jan 01, 2019 9:24 pm

When an antenna is mounted above the ground, the radiated signal bounces back to the antenna and either adds or cancels, depending on the phase. This in turn changes the impedance of the antenna.

If the antenna is mounted less than 1/2-wave (when addition occurs), then there will be a drop in impedance, but the first addition will occur, not vertically upwards, but at an angle to the vertical.

This is true for a horisontal dipole, but if you are using a rectangular loop then the radiation from the antenna will not be uniform. That is to say that the effect of the ground will be less if the antenna radiates more in the horisontal plane than the vertical.

As regards eliminating reactance, I would try without any adjustments. If necessary then you could either use capacitive corrections or use a different feed method, for example a balun transformer:

8 turns on a ferrite ring.
Turn 0 = antenna feed
Turn 4 = antenna feed
Turn 2 = TX coaxial ground
Turn 6 = TX 50 ohm feed

50 0hm feed can be changed from turns 4, 5, 6, 7 and 8 to give you a range of impedances from about 25 ohms to 150 ohms at the antenna.

This of course is if I have correctly understood your arrangement ;-)

BR and HNY from Harry - SM0VPO

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rectangle antenna for 10m Empty rectangle antenna for 10m

Post by Bernie on Wed Dec 19, 2018 10:41 pm

I am nowadays thinking to build a rectangle full wavelenght wire antenna for 10m.
The design has 1/3 wl height and 1/6 wl width.
It is said to presente "50 ohm impedance" if fed at middle of the base.

Then I saw in another article a chart showing its impedance will in fact be function of the distance to ground.
Let us say, if lower side is 3m above ground the impedance is estimated to be 30+30j.

My doubts:

+ What is best: to eliminate the reactance with a capacitor or tuner to have 30 ohm resistance, or leave it as it is (30+30j) about 45 ohm impedance in modulus. My transmitter (homebrew qrpp) has a 3 element 50 ohm pi network at the output.

+ I dont know even what to measure to choose the best height. Is it to stop at the higher reading of a field strenght meter?

+ If so, any worries about the transmitter output transistors? Just 2x2n3904 in parallel. Power about 100 mW.

Thanks.

Bernie

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