The SHORTY MULTY

My new antenna used with good results on 40m, 20m, and 15m. Here is the schema.

 

The antenna in the origin was an OCF (off center feed dipole), of 21 meters; this is a simulation of the OCF (not linear loaded):

The OCF was too long for my balcony, so i tried a new configuration: now it is a 23,4 OCF dipole, linear loaded. The center of the cable is attached to the vertical part, that is the aluminium tube.

I use it with my micro Bitx, with 10 Watt on 40 m, 3 watt on 20 m and 1,8 watt on 15 meters with very good result (in 20 m and 15 meters i work QRP, with less of 5 watt !!!). I use a Kenwood AT-130 antenna tuner to tune it.

In the next post i will report my testing.

73 de IZ3AYQ

OFF CENTER FEED DIPOLE: A SIMULATION AND A TEST

OFF CENTER FEED DIPOLE:  testing unusual % dimensions

Total length: 21 meters; Off center feed at 25% of lenght (usually we can find 30%)
One leg: 5,25 meters; the other leg: 15,75 meters.
Why 5,25 m? This is not the best %; but for the moment i do not want to cut my vertical antenna aluminium tube. This is a test; if a will have good result i will try to optimize.

You can see minimum SWR at 7 MHz, 14,2 MHz, 21,1 MHz an 28,3 MHz. Using it with a balun and/or wiht an antenna tuner you can easily tune it for these band.
So, i am going to test this antenna with my RIG:

MicroBitx with antenna tuner Kenwood AT-130; the antenna has  a vertical 5,25 meters tube, and a wire of 15,75 meters that is over a tree in his final part.

Today easy contacts on 40 meters, and first contact on 60 meters (with SWR at 1:1.1)

73 de IZ3AYQ

Off Center Feed Dipole: some simulations

Here are some simulations for OCF (off center feed dipoles) of about 20 meters:

The original antenna was developed in 1929 by Lorenz Windom, W8GZ. The orignal antenna was worked against ground. During the years, similar antennas were developed and are generally called Off Center Feed Dipoles. They are simple to build: a simple wire and a balun 4:1 or 6:1, to convert the impedence of 200 - 300 ohm to 50 ohm.
Interesting to study in the Internet: windom, carolina windom, ocf dipole, double windom, balun.


A) L= 20.70 m, L2=6.70 m (L2/L=32%)
The OCF dipole can work (with balun 4:1 or balun 6:1 and tuner) on 7, 14, 28 MHz.

B) L= 20.74 m, L2=6.95 m (L2/L=33%)
Similar to case A): only 3 bands, 7, 14 and 28 MHz.

C) L= 20.1 m, L2=3.7 m (L2/L=18%)
 The OCF dipole can work (with balun 4:1 or balun 6:1 and tuner) on 7, 14, 21, 28 MHz.
We have 4 bands!

D) L= 20.2 m, L2=6.7 m (L2/L=33%)
 Similar to case A): only 3 bands, 7, 14 and 28 MHz.

E) L= 20.70 m, L2=3,8 m (L2/L=18,4%)
 The OCF dipole can work (with balun 4:1 or balun 6:1 and tuner) on 7, 14, 21, 28 MHz.We have 4 bands! This is the best result!

 73 de IZ3AYQ