DIY antenna

[Fry-kun]

Hi there,
I'm trying to make an FM antenna for my receiver (Onkyo TX-SR604) using these instructions.
Hopefully it'll be enough for my needs... though it looks like I need a 300->75 ohm converter (and apparently those decrease the signal quality)

Anyway, my question is more general: it seems from the antenna design that the wire is looped - which raises the question "where does the 300ohm come from?". If the wire is in a loop, the resistance is barely anything... or am I missing something?


TIA for any info

[DigitalJunkie]

Yea,the DC resistance will be quite low,but the impedance at ~100mhz will be somewhat higher (somewhere around 300 ohms.)

[Kent Smith]

A little more info....

The antenna impedance is a combination of the resistive loss (similar to DC resistance, but not exactly) and the radiation resistance, which is the load caused by the actual radiated power.

The basic antenna is a halfwave dipole. Impedance is 75 Ohms. To find the length, divide 5904 by the freq. in MHz. About 60 inches for 98 MHz. Cut the wire in half and feed coax to the middle (center to one side, shield to the other). Good antenna, but it does have a null along the axis of the wire.

A Loop or Folded Dipole (basically a squashed loop) will split the current between two halfwave sections, thus causing the feedpoint impedance to increase by 4 times; 300 Ohms. A wonderful thing when using 300 Ohm twinlead, but not so great for coax. The losses in a 300 to 75 Ohm transformer will probably eat up what little you gain in the loop. And the loop will still have the same polarization and null issue.

And, Yes, I do work in the field. I love antennas....

Have Fun,
Kent

[cyclotronguy]

I Vote J-Pole... or several 1/2 waves in phase.

Just about as easy to build


Cyclotronguy