__The
Geordie/Edme/John Bull Brewtin Antenna__

__NOTE PHOTOS WILL BE ADDED SOON!__

This little antenna/dish feed is delightful. There's been a lot over the years on plumber's delight. Surely, this little gadget must be a brewer's delight! Of course, you will need the following to get the most from this antenna....

One five gallon brew bucket

The required amount of sugar

A copious amount of beer bottles and corks sufficient for 5 gallons of brew

You can use tins from Geordie, Edme, or John Bull kits. I particularly recommend the 'Scottish Export' from Geordie, as the quality of the brew is excellent. Sorry, but barley wine tins aren't really long enough!

The beer aside, carefully remove the tin lid from the tin. You can use the old fashioned tin openers or the new type which cut the lid and crimp clean off. I recommend the old fashioned type however, as the crimp ring gives the tin a little more stability. Make sure you've washed every last trace from the tin, and remove any paper labels. Painted tins are fine, and can have the necessary area sanitised by use of a little paint stripper or wire wool.

Much has been written on circular waveguide (CWG) antennas for the microwave bands. The theory is to excite the TE(H)11 mode in the tin. Placing a quarterwave probe (freespace length) inside the tin at a distance of a waveguide quarterwavelength from the closed end.

The 'guide' must be large enough to accommodate the TE11 mode in the first place, and

Lambda C (cutoff) for the TE11 mode is 1.706 * Diameter, which implies a minimum CWG diameter of 0.59 Lambda

Now Lambda C (TM01 Mode) requires the diameter to be at least 0.76 Lambda. Any cylinder (or CWG) large enough may support the TM01 mode - but not really required as it is the dominant TE11 mode we wish to encourage, and launch.

**The waveguide formula...**

Lambda g = Lambda / (Square root of (1 - (Lambda / Lambda C)^2))

**Mode formulas for TE11 TM01 and TE21
modes...**

__Cutoff Wavelength (Lambda C)__

**(TE11)** W = 1.71 * D

**(TM01)** W = 1.31 * D

**(TE21)** W = 1.03 * D

__Cutoff Frequency__

**(TE11)** F1 = 30000/(1.71 * D)

**(TM01)** F2 = 30000/(1.31 * D)

**(TE21)** F3 = 30000/(1.03 * D)

D is the diameter of the CWG in *CM*

The best choice of CWG diameter is when the operating frequency falls between the TE11 and TM01 cutoff frequencies. This prevents multimoding, and VSWR difficulties aggravated by this. Always keep the operating frequency below the TE21 cutoff, TM01 moding isn't half as much a problem as TE21 moding is.

so..

*(i)* Calculate the Diameter of a CWG for TE11 cutoff
15% lower than the lowest frequency in use (this gives smallest
usable diameter)

*(ii) *Calculate the Diameter of a CWG which gives TE21
cutoff 5% higher than your highest operating frequency (this
gives the largest usable diameter)

**The usable diameter is anywhere between these two
limits.**

As an example. A CWG antenna was required to cover a centre frequency of 2.365GHz +- two adjacent channels occupying a 20MHz bandwidth each after consideration of guard bands, The lowest frequency of operation was 2347.75, and the highest 2382.5MHz.

TE11 cutoff should be @ 14.6969cm or 2041.2467MHz. Diameter is 8.59cm.

TE21 cutoff should be @ 11.961cm or 2508.1514MHz. Diameter is 11.6126cm

Can size can be between these two limits from 85.9 to 116.13mm diameter

**Lets get away from the
maths - beer is more interesting**

For the Geordie tin. Diameter is 100mm - perfect. The length of the tin is ideal too, approaching LambdaG itself.

We calculate the TE11 cutoff at 1754.386MHz, the TM01 cutoff at 2290MHz, and the TE21 cutoff at 2912.621MHz. Ideal!

So to calculate the probe for the 2365 centre frequency. Lambda (freespace) is 12.685cm

We then work out Lambda G for the CWG and find it to be 18.9cm

Divide this by four, and that is our drilling point on the tin for the quartwave launcher.

We take the tin, and find the inside bottom with a depth probe (vernier will do this, or a steel rule). We then subtract the quarterwaveguide wavelength from this and mark the position on the tin. This is where we will spot through for the probe. We'll use an SMA socket, as an N type is not really mechanically suited to this size of tin. If you can make a brass saddle to sit the N type on and solder it to the tin, all well and good - go ahead and use an N socket. Solder it in if necessary and happy with your soldering, otherwise use nuts and bolts and keep a minimum of metal inside the guide - recommend the bolts are fitted with their heads inside the CWG, and the nuts/washers on the outside. Don't for heavens sakes use pop-rivets - it'll distort, as well as making for dodgy joints in the future!

Once done. Solder in the launching probe. Virtually any stiff wire to fit the spill will do. Measure the VSWR at LOW POWER and for goodness sakes don't go looking into the guide, or wafting it around at anything living. Straight up and far away will do nicely. If you have the facility you can then tweak the positioning of the probe somewhat to reduce any reflected power to a minimum, and that's it. I used 100mW to tune mine with. That was sufficient to get a reading on reflected power of a couple of milliwatts, and in the end reflected power was reduced to a milliwatt. A quite good match really. If you can't measure the VSWR don't worry. If you've followed everything to the letter you will not be far off.

In operation this thing is really quite a performer. It's good on it's own, you can add a flared funnel to it if you wish, or bolt it up as a dishfeed. The beerkit comes with it's own radome - a nice plastic cap which snaps shut over the open end - keeps the spiders out!

Of course, now it is made, you have 5 gallons of very nice beer to look forward to. It's better if you don't make too many of these in one go!

This 'antenna' is also used as a circular waveguide launcher in order to retune the calamp 13000x integrated downconverters, the unit is placed over the open end of the brewtin and rotated for best signal (polarisation match) before alignment. Similar techniques are used with 10GHz LNB's to allow alignment without RF wafting around the room and stray pickup - see the 10GHz page when it's finally built - It's so frustrating to have six years worth of intensive handwritten notes needing to be turned into html!

AM 24/07/02

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