Monday, October 22, 2012

Dissection of Icom FA-S6270A antenna

In this photo you can see a dissection of a quad band Icom FA-S6270A rubber antenna who suddenly stopped working:

This is the antenna supplied wit the Icom IC-T81 quad band walkie talkie. The symptoms were very evident: The antenna suddenly become deaf, giving very poor signals, so it was sacrificed in the name of science.
Once removed the rubber cover, it is easy to see it is a very classical design.

The B section is the main antenna. It's an helicoidal section. A is the loading coil who makes B resonant on 2m band. On 70cm band B is also the main antenna, but now it is capacitively loaded by A. The coil is almost a RF choke at 70cm band, but because it is wounded with no gaps between turns, stray capacitance makes it to look as a capacitor at UHF frequencies.

A and B makes a classical 2m/70cm dual band antenna. C is a top load coil, made by very thin wire wounded all together who makes AB resonant on 6m band. Because its high inductance it has no effect at 144 or 432 MHz band, producing a three band antenna. This is the same concept used by Yaesu with the FT-817 rubber antenna, except the 50 MHz section can be removed.

The large C coil is the reason the antenna has so high Q on 50 MHz band and resonance is too sharp, making almost a useless antenna for this band.

Are you missing something? No? I do.

I miss the 23cm section of the antenna. There is no dedicated 1.2GHz section in this antenna, and this explains the poor results of this antenna in 1.2 GHz band. At 23cm band the working part is AB, just like at 432 MHz band. At 70cm band the antenna works as a quarter wave antenna, and therefore, it works as 3/4 wavelength antenna at 1.3 GHz. But it is an helicoidal antenna, so lobes are far from satisfactory.

It is interesting to see how the antenna is named. The model is FA-S6270A. The dual band antenna supplied with the IC-E91, IC-Q7, IC-R2, and many other dual band devices is FA-S270C. So it seems Icom named the antennas according to the bands they work in. A 2m/70cm antenna is named FA-S270C, so it's sure FA-S6270A is named for the 6m2m and 70cm band. Where is the 23cm band? The antenna should be named FA-S627023A but now it is clear this antenna never was intended for the 1.2 GHz band.

If you own a Icom IC-T81 and want acceptable performance on 1.2GHz band, just replace the antenna with a real 1.2GHz antenna.


  1. Nice analysis, Miguel. But, did you fix the antenna ? :) :)

    1. No. The failure was the connection between the SMA pin and the first coil, inside the plastic block. Sure it was a broken soldering. 73!

  2. This is a nice post about Icom IC-T81 quad band walkie talkie antenna. its also nice analytical and helpful post.