Multicouplers for AM and SW reception

Hi,

I have several shortwave radios, but only one antenna, and wanted to use them without having to continually disconnect and reconnect the antenna. Simply connecting the sets in parallel produces unfortunate interactions between the RF tuned circuits (at least in those sets that have them!).

A solution is to use a multicoupler. Constuction details can be found at

http://www.dxing.info/equipment/rolling_your_own_bryant.dx

The following is a photo of my implementation. I tested the unit using a signal generator and 'scope to make sure the transformers were all working. Final tests were done using an AR88, R-390A and Drake R8. This is a handy gadget for comparative listening tests.

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Bryce, thanks for sharing this useful device that is particularly useful for collectors of many radios.

One thing to keep in mind is that if all impedances are perfectly matched and transformer loss is negligible, the available signal power is cut in half (-3dB) for each of the radios.

I setup a similar splitter for the two FM radios in the two bed-side tables in my bedroom. It worked as expected, but I had to use an FM preamp to make up for the -3dB loss in the radios.

The two radios are tube radios, so I ended up using an 1950's Astatic BT1 VHF TV preamp that included the FM band. If memory serves me the preamp gain is in excess of 10dB in the FM band.

I also built an external current sensor for the mains power of each radio that drives a relay to turn on the tube preamp, so all you have to do is to turn on either radio to get the preamp turned on.

A preamp with 10dB gain can supply signal to 3 radios without any loss, except that the normalized signal will have added preamp noise. The best situation is to have a higher total gain in the preamp than the total number of sets needs, at -3dB per set. The higher level signal at the radio means that only the noise of the preamp will be heard, instead of the noise of the preamp co mbined with the noise of the radio. This is particularly important at FM.

The SNR advantage of the preamp is not needed in the lower SW and AM bands, where natural atmospheric noise dominates. The only thing that is lost is the -3dB gain, but this modest loss will certainly be absorbed by a high gain receiver, and tolerated in a simple AM radio because the external antenna is already providing a lot of signal.

Comments invited, best regards

-Joe

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