sirio gain master homebrew

| agosto 14, 2015
Homebrew Gain Master using RG58 and 2.5mm wire.

REV2: Updated 15:20 04-01-13

Evening all.
Am reporting here my findings on my latest project. Homebrewing a Sirio Gainmaster antenna using commonly found bits and bobs found in the shack.
Did it work? A tentative yes. It tunes – so the theory holds up well and is quite forgiving. Proper field testing will be done on the weekend as I ran out of time today and sadly I am in work tomorrow.

The Sirio Gainmaster is a broadbanded 5/8 wave centre fed “sleeveless” sleeve dipole antenna. A sleeveless sleeve antenna is defined as a sleeve antenna that uses the parasitic common mode current on the outside of the shield in the coax as the lower half of the dipole. To stop the rest of the coax radiating and causing tuning issues a choke is immediately placed after the desired length of the lower half of the dipole to eliminate any common mode currents getting further back down the feeder coax and back to the shack.
An 8.6 PF Capacitor is used at the base of the upper radiator to give a useable tunability of between 25.5 to 29.6 mhz This is a unique feature of this antenna.
A 5/8 wave antenna has a feed point impedance of approx 200 ohms this for us 50 ohm users equates to a VSWR of 4 to 1. Niether use nor ornament. So a shorted matching stub is placed in the feedline approximately 1.91 meters multiplied by the coax velocity factor below the centre (feed point) of the antenna to match to a palatable 50ish ohms. This again is a unique feature for a sleeved antenna.

Some musings about the matching stub and the attaching point. I found the match point the old way. test, adjust, test (lather, rinse repeat). However once I had the match point I was able to crunch some numbers regarding the match point. The Match point of the stub is 126.5 cm away from the feed point using RG58.
This multiplied by the coax velocity factor (0.66) gives us the free space distance which is 191.666666667 cm
Now consider this:
Using the original match point of 149.5 and dividing it by 191.666666667 gives us the velocity factor of 0.78 for the original coax used by the Original Manufacturer of the design. I know that RG8X can be bought with a VF of 0.78 and with a solid core centre element. So I wonder if that’s what was used in the original? Food for thought.

Conversely this also means that if using a different coax with a different velocity factor (VF) you can calculate the matching point connection using the formula: 191.666666667 * VF.
EG: RG303 with a VF of 0.7.
191.666666667 * 0.7 = 134.166 cms from the centre feed point.

Stub length can be calculated from the original as we know that Rg303 was used for this and thus:
59.5cm / 0.7 vf = 85
For our RG58 Stub with a VF of 0.66:
85 * 0.66 = 56.1 so our stub length is 56.1 or there abouts. Simply trim this to get the best match. Mine trimmed to 55cm.

Homebrewing Sirio Gainmaster

Notes: The above is measured out using Belden RG58 coax with a velocity factor of 0.66. If you use a coax with a different VF then the stub connection point will change as will the length of the matching stub.

Below is a picture of the actual dimensions of my freshly fabricated poverty spec Gainmaster. This was using Belden MRG5801 (the new name for RG58 from Belden) and 2.5mm (14AWG) stranded insulated copper wire for the top half of the antenna.

Walking through the construction from bottom to top:
Coax Choke: Find a former 66mm in diameter. Wind 16 turns of coax on it and secure it properly and tightly. If you have used a conductive former remember to remove it. Then Do not cut the coax! Measure a length of coax 360cm from the top edge of the Coax choke. This is where you want to cut it.

Matching Stub:
Measure a length of Westflex 103 coax 75 cm and cut it. Now strip one end 25mm so that you have the centre core and the shield seperated into 2 tails. You need to attach this centre core to centre core and braid to braid at a point 218.5 cm up the coax from the top edge of the coax choke. How you attach it is up to you. I prefer the cut and splice method personally. If you feel that 25mm of tail s is too long feel free to trim them to suit your build. Just remember to keep 72cm of coax untouched as this is the business end of the matching system.
On the unattached end of the matching stub strip it back 5mm and short it out braid to centre core and tape it downwards to the main coax.

Coax Capacitor:
Measure up the main coax from the top edge of the coax choke 345cm. Now mark this point. A further 2.5cm up the coax mark again. You need to strip away the outer sheath and the coax braid between the 2 marks.
Now measure up again 8.64cm from where you have stripped away the coax and strip the outer sheath away. Split the shield and twist back to form a solder tag. The inner coax core that is protruding can be trimmed flat but leave 6mm or so protruding from the end. Insulate and weather proof the centre core.

Homebrew Gain Master

Top Radiator:
Solder a length of 2.5mm wire to the solder tag you created. Now measure 345cm from the centre point of the antenna (The point where we stripped 25mm of the sheath and braid) and cut the wire.

Now erect the antenna on your favourite non conductive pole and tune by trimming the end of the matching stub and shorting it out again. when you’re happy, weatherproof. and attach it permenantly to you favourite non conductive pole.

Here are the results from my trusty MFJ antenna analyser.

25.6 Mhz: A bit low ohms wise but still within the 2:1 limit

27.5 Mhz: 1.3:1 Just where I like it.

28.5 Mhz: 1.1:1 To be honest I expected the lower ratios to be a bit further down the band. Hmmm

29.6: 1.6:1 It’s worth noting that the broadband VSWR curve shoots upwards a lot quicker at the upper end of the frequency as the antenna become too long.

As a foot note all pics and hi-res available here:

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