Carndonagh Amaterr Radio Club

Beer Barrel VHF Duplexers

G8PWE & EI4JR

In the UK and other places there are becoming available on the scrap market Beer Barrel Cavities. We know that these have been used on frequencies of 80MHz and 150 MHz. The differences between high and low band barrels is a simple cup that is fitted in the bottom of the low band version. This is removed as we are building a VHF version.

I'm sure that there are other methods that can be adopted in this modification. This is just what I have done in this build and seems to work very well.

The original modification design for the use of the beer barrels was by Ian G8PWE with a little help from Peter EI4JR

The Beer Barrel as a VHF Cavity Resonator. The Concept (PDF)

Click on pictures to enlarge them

The Barrels are 400mm Diameter by 530mm Tall. The pictures above are before modification. Aluminium was used in the manufacture of the barrels. The centre plunger is silver plated brass The inside of the barrels are silver plated through out.

Removing the 8 set pins enables you to remove the silver plated plunger as shown. Be very careful of the finger stock not to damage it when removing and inserting the plunger into the barrel..

Inside the Barrel you will see the cup that needs to be removed if fitted (6 small set pins hold the cup)

Cup is removed. This is no longer needed

We need to make both coupling loops rotatable. Normally only one is adjustable. Remove both coupling loops. Make the fixed loop connection hole the same size to the same size of the the rotatable hole. A good round rough file will do this with ease. Scrape away the paint round the hole you have made bigger to enable a good contact with the new adjusting coupling loop rings. A vacuum will be needed here to remove and filings that will fall in the barrel when you have finished filling.

Using the rotatable coupling loop rings as a pattern make a copy this will be used for the new rotatable coupling loop. I used an old aluminium 19 inch rack 3mm thick panel where I marked and made the rings from cleaning any paint from the rings as to make good contact. You will need to do this for all barrels.

Unsoldering the old original loops from the "N" type socket. Drill and connect the "N" type plug with small countersink set pins. I used 6BA brass set pins but I'm sure you will use what you have.

The second ring I secured to the smaller ring using a self tapping screw. Here you can drill and tap and use small set pins instead if you wish. Again there is no hard and fast rules here as long as it works like the original rotatable loop rings.

Using the chart below make the coupling loops from 2mm silver plated solid wire. 2 for each barrel. You may find some of this silver plated wire in some coax cables that you can use. I used a small eyelet soldered on the earthy side of the loop and bolted to the coupling ring as shown above. We have tried various sizes of loops and found the ones shown to be the best.

The Coupling Loops are made from 2mm silver plated wire.

I used Thermax RGU 142 Coax cable (50 Ohm) for making up the interconnections between the barrels. It has not got the best loss figures but it is silicon based and will not melt when soldering the coax especially to the tin screening cans. I'm sure that you will use what you have. Just keep in mind about the heat and melting the coax.

The "Circulator / Isolator" I used was a Sinclair Dual Ferrite Isolator. This is used to enable the TX and RX to use a single antenna. You may have other ideas in this area.

The screening cans measure 50 x 50 x 25.

A 7mm hole in the side is needed for a trimming tool to pass through so as the tuning capacitor can be adjusted as shown also cut out 2 slots as shown 12mm wide to enable the coax runs to pass through.

2 fixing holes are need to the fix the screening can to the thick plunger housing. I used self tapping screws here but I'm sure you will drill and tap and use small set pins.

A good vacuum cleanout will be needed here to remove and filings that have fallen into the barrel.

Thermax RGU 142 Coax was used for making up the interconnections between the barrels. It has not got the best loss figures but it is silicon based and will not melt when soldering the coax especially to the tin screening cans.

A 12p variable trimming capacitors is soldered across the centre of the coax making sure that the capacitor does not short to earth. This capacitor is used to move the notch

Do not have the screening can secured to the plunger housing as this would take the heat away when soldering the coax screens to the screening can. You will need a high wattage iron to solder to the screening cans. I use a 65 watt Iron for this job. You will need a helper when doing this as the coax has a mind of its own.

Make up the 2 sets of coax harnesses one for RX and one for TX. I used right angle "N" type crimp on plugs and found them very good. Make sure that you have the plugs at the right angle before crimping. so they fit the sockets when the screening can is in place.

The "Circulator / Isolator" (Shown Above) I used was a Sinclair Dual Ferrite Isolator. This is used to enable the TX and RX to use a single antenna. You may have other ideas in this area. If you are going to use a 2 antenna system then the circulator / Isolator will not be needed

2 Barrels used for RX and 2 for TX

RX . Pass = 145.1125 MHz - Notch = 145.7125 MHz TX . Pass = 145.7125 MHz - Notch = 145.1125 MHz

RX 95dB 2dB loss & TX 95dB 2db loss including the Circulator / Isolator

Tuning the duplexers may take a little time. Tune one set (2 Barrels) at a time. Set the tuning capacitor to nearly minimum capacitance. Connect the set to a spectrum analyzer with sweep generator set for the RX frequency receiver. With the RX connector to the input to the analyzer and the circulator connector to the output of the analyzer. Slide the plungers so as the pass peak waveform is achieved in the display as in the above picture. You will find 2 pass peaks as you are adjusting the plungers. These peaks should merge together on the RX frequency as you push the plungers and adjust the loop in turn until you have the best pass peak you can get. Rotating the loops you will achieve either a peak or a notch. It will take you a little time when tuning so be patient. Your notch will be at the frequency of the repeater TX frequency. The 12p tuning capacitors will fine tune the notches after the coupling loops have been rotated to get the best notch.

Do the same with the TX set of barrels except the peaks and notches are the other way round this time and rotating the input and output connections to the spectrum analyzer

HP 8594E Spectrum Analyzer and Marconi 2955a Test Set Used during the tune up

I made a simple beer barrel rack to hold the barrels and a cabinet to hide them away in the shack

Ian G8PWE doing his magic with the final tune up

All done and working under test with the EI2IPG Repeater

Ian G8PWE & Peter EI4JR

Webmaster Peter EI4JR