T antenna with matching network

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T Aerials Drawn by Roger G4SEF

T Aerials With Matching Network

Electrical Notes

This T aerial broadcast design by Guglielmo Marconi has been built and is used by Jim NN4AA on 160m, all details and drawing are copied from Jim’s Notes he sent me.

As Built,The aerial will Resonate at 1640khz
This raises the radition resistance about 1 ohm over what would result from a resonant configuration.
If the aerial is built higher or longer, a still lower resonate freq will result.
Radiation Res. is 11.5 ohms current in the down lead is about 2.3 amps at 100 watts
Gnd resistance is about 6-7 ohms.

Tuning notes

  1. Put the matching network at the bottom of the aerial where the ground radals terminate.
  1. The aerial is fed at Jim’s QTH via 400 foot of underground 1/2 inch hardline.
  1. Since the aerial resonates well below the band, it was tuned up to 1860khz by means of a 1600pf fixed capacitor in series with the down lead.
  1. Resonance can be determined by connecting the tuning cap directly to ground via a short clip lead. A grid dip meter can then be coupled to the clip lead. A 2 turn loop maybe needed here to get adequate coupling
  2. Matching L design is then designed to match 50 ohms to 18 ohms for perfect vswr at 1860khz. 1.5:1 BW is 45khz, 2:1 is 90khz.
  1. You do not have to duplicate the aerial dimensions exactly, with a shorter flat top resonance could be achieved at your operating frequency. in that instance the series capacitor would not be needed, but you would still need the L network.

Matching Network

 

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