Rotary Coaxial Cable Connection Experiments

Overview

From time-to-time the amateur SIGINT enthusiast may require an antenna system that is fully rotatable and immune to tangled coaxial feed lines.  The usual solution to this problem is to purchase a commercial rotary in-line coaxial cable adapter, which actually do exist.  The problem with these adapters is that they are expensive, very hard-to-find, and may also be frequecy limited.

Our solution is to build our own using only commonly available parts.  Our rotating coaxial adapter will be built around the spinning ferrite transformer inside the rotating drum head in all VHS VCRs.  Every VHS VCR in existence has this type of rotating head, so supplies should be no problem  The rotating VCR drum head's construction and operation is quite ingenious.  A small coil of wire, which makes up the actual playback/record head, is located inside the VCR's drum.  This head (2 or 4 per drum) is used to read or write the low-frequency signal on the VCR cassette tape itself.  Since the drum is rotating at around 1,800 Hz, and in order to transfer the signal from the heads to the rest of the VCR circuitry, the drum itself is made up of two rotating ferrite-coupled transformers.  The head's schematic looks just like any 1:1 coupling transformer, just that the core is capable of being rotated a continuous 360 degrees.

The main drawback to using rotating VCR drums is their poor high frequency response and high insertion loss.  Testing showed a limited frequency range below 10 MHz.  This should be O.K. when used in downconverted Intermediate Frequency (IF) applications.  Also, the drum's coupling loop windings were not impedance matched to 50 ohms, which probably accounted for most of the signal loss.  With a little tweaking, a series-shunt, dual-capacitor impedance matching network could be used to match the transformer to 50 ohms over a limited frequency range.

In the below application example schematic for the Rohm BA7172 2-Channel Video Signal Amplifier IC, you can see that the VCR heads are actually little transformers with their core being rotating ferrite material.  Note that the two VCR heads share a common output connection.  These wires will need to be unsoldered and isolated for this experiment.

Construction Notes & Pictures

Several different types of VHS VCR rotating drum heads.  These are from VCR models made between 1993 and 2000.  They'll all probably be very electrically and mechanically similar.

Get a whole bunch of old VCRs to play with and take apart.  They are filled with lots of nice, salvageable components.

Bottom view of the drum.

The drum's motor windings and electronics can be removed.  They are usually mounted to the spindle with two screws.

Top view of the drum.

The bottom drum section is meant to be stationary while the top drum half, which contains the actual playback/record heads, spins at a rate of 1,800 Hz.

Separate the two halves.

Note that each half contains a loop of wire mounted in a piece of ferrite material.  This is what makes up the actual rotary transformer.

There is usually an Allen screw and clamp on the spindle that will need to be removed.

Closeup of the top rotating half.

The actual playblack and recording head windings are soldered to the little brass plates on each side.  This is a two-head VCR design.

Closeup of the bottom stationary half.

Note how the two separate windings share a common connection.

One of the coils should be removed completely from the head to avoid any unnecessary circuit loading.

Closeup of the actual playblack/recording head's two solder connections.

Unsolder the two wire connections and remove the brass plate.

Solder some coaxial cable to the wire loops on each of the drum sections.

No impedance matching will be used for this experiment, but you should match each of the loops to 50 ohms to lower the insertion loss.

Also, note that one of the coil windings (outer) on the ferrite core has been removed.

Completed rotating coxial connection using an old VCR drum head.

The insertion loss was quite high, and it only works up to around 10 MHz.

Test the connections by attaching the rotating connector to a HF receiver and monitoring the NIST time standard signals at 2.5, 5, 10, 15, and 20 MHz.

Proper 50 ohm impedance matching to the loops would probably increase the overall performance significantly.  The operating frequency range will probably still stay low though.

A video of the adapter's rotation in action is available here: http://zine.gbppr.org/Rotary_Coaxial_Cable_Connection.wmv












Sharp Four-Head VCR Schematic