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Homebrewing a Sirio Gainmaster using RG58 and 2.5mm wire.

Posted: 03 Jan 2013, 21:15
by PsiDOC
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.

Introduction:
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.
Broadbanding:
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.
Matching:
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.

Construction:
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.

Image

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.

Image

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.


Tuning:
Here are the results from my trusty MFJ antenna analyser.

25.6 Mhz

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

27.5 Mhz

Image
1.3:1 Just where I like it.

28.5 Mhz

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

29.6

Image
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.

And Finally:
A shot of the antenna in the test location.

Image

As a foot note all pics and hi-res available here: http://www.psidoc.com/gainmaster/

Hope this is of use to anyone.

Psi

Re: Homebrewing a Sirio Gainmaster using RG58 and 2.5mm wire

Posted: 03 Jan 2013, 21:30
by FAFFER
Awesome!

Have you got another antenna to compare it against? As in, a silver rod at the same qth...

Re: Homebrewing a Sirio Gainmaster using RG58 and 2.5mm wire

Posted: 03 Jan 2013, 21:37
by PsiDOC
The only thing I have is G5RV deriative (ZS6BKW) running east / west. On 10/11m it gives 4 defined lobes of about 5dbi towards Lower North / S.America and Norway and over the middle east. This is the only base point I have and I will be using it.

Psi

Re: Homebrewing a Sirio Gainmaster using RG58 and 2.5mm wire

Posted: 03 Jan 2013, 22:41
by crusty
Nice work. What did you use for the the tapered antenna sleeve ?

Re: Homebrewing a Sirio Gainmaster using RG58 and 2.5mm wire

Posted: 03 Jan 2013, 23:07
by Irishman1127
Telescopic fibreglass pole I'd say....

Re: Homebrewing a Sirio Gainmaster using RG58 and 2.5mm wire

Posted: 04 Jan 2013, 00:13
by MrWeetabix
Great work....... when can we buy them? :-p

Re: Homebrewing a Sirio Gainmaster using RG58 and 2.5mm wire

Posted: 04 Jan 2013, 06:22
by PsiDOC
crusty wrote:Nice work. What did you use for the the tapered antenna sleeve ?
29GD01 is right on the money. It's an ebay special 10 meter fishing pole that cost £25.
Also bear in mind the antenna will be always be cable tied to the outside of the pole. Trying to get it inside with the coax balun outside will weaken the pole. Weather proofing it externally shouldn't be a problem anyway.
MrWeetabix wrote:Great work....... when can we buy them? :-p
From me? Probably never mate. I'd rather fire up peoples creativity. A couple of hours work with some wire and coax and you can make your own. The hard work - finding the correct point for the matching stub - is done for you. ;)

Will post full construction details later today. Off to work in a moment sadly. :(

Psi

Re: Homebrewing a Sirio Gainmaster using RG58 and 2.5mm wire

Posted: 04 Jan 2013, 11:36
by crusty
Ah, wondered what the banding (tape) was.

Re: Homebrewing a Sirio Gainmaster using RG58 and 2.5mm wire

Posted: 04 Jan 2013, 11:40
by Ashtec
Intresting ;)

Good work!!!

Re: Homebrewing a Sirio Gainmaster using RG58 and 2.5mm wire

Posted: 04 Jan 2013, 15:25
by PsiDOC
crusty wrote:Ah, wondered what the banding (tape) was.
merely insualtion tape that I had handy to attach the antenna to the pole. ;)

Psi

Re: Homebrewing a Sirio Gainmaster using RG58 and 2.5mm wire

Posted: 04 Jan 2013, 17:06
by wa10
nice work psi, glad you got it working ;)

Re: Homebrewing a Sirio Gainmaster using RG58 and 2.5mm wire

Posted: 04 Jan 2013, 17:46
by bigpimp347
good way to save a few quid..
i made a double bazooka and am thinking about sticking it inside a plastic tube,
same could be done for a psiDOC gainmaster simple plumbers pipe and some joints and sealer and you have a pretty sturdy aerial..

Re: Homebrewing a Sirio Gainmaster using RG58 and 2.5mm wire

Posted: 04 Jan 2013, 18:17
by PsiDOC
Gainmaster indeed ya cheeky **** Mark!

Although, yes you are correct, although at 24 feet tall the pipe and end connectors would have to be very sturdy.

Psi

Re: Homebrewing a Sirio Gainmaster using RG58 and 2.5mm wire

Posted: 06 Jan 2013, 10:24
by PsiDOC
HI all. Just a follow up on the build.
The antenna has been up since friday night, however as usual the upper portions of the band were closed yesterday with nothing heared on my main antenna or the homebrew. Sadly local 27mhz around here is as dead as a dodo and has been for a couple of years now. So no local contacts to test with either.
Due to mother nature not being on my side yesterday I did some TX tests and then tried the antenna on some lower bands with a tuner.
TX Tests.
Burn in test: 100 watts dead key on 28.250 for 5 mins - No flashovers and no fluctuations in SWR.. I think we can call that a pass.
Swr tests @ 100w, antenna 2.8 meters above ground:
25.5: 1.9:1
26.0: 1.3:1
26.5: 1:1
27.0: 1.1:1
27.5: 1.4:1
28.0: 1.4:1
28.5: 1.1:1
29.0: 1:1
29.5: 1.4:1
30.0: 1.9:1

Interestingly I now have the double null in the swr curve that the original has, however when I was testing with the MFJ at the feed point I was not getting this, which puzzled me a bit. I guess the feedline has some bearing on this.

Common mode current (rf on the feedline) test @ 100w:
Zero, nil, nada, nothing. The coax choke seems to work well!

21mhz test @ 2.8m above ground.
Seeing as the 12, 11 and 10 meters were lifeless I thought I'd try my luck with it on 15 with the tuner. Bearing in mind on 15 the antenna is approx a 1/2 wave so it should work reasonably well.
I does indeed work with a few contacts into Russia, namely moscow on 50 watts with the obligatory 5/9 so after explaining I am testing the antenna and would they be so kind as to recheck the signal report, a more realistic 5/6 was reported. They were 5/7 to me as at 100w.
So... it's not a dummy load and it does work even a bit out of band.

Today's testing @ 2.8m above ground:
Finally 11/10 are open a bit!
Currently working eastern europe with repeated 5/9 (why do people do that?) and when asked to reconfirm am getting 5/6 reports using PSK at 20w. Switching to the wire antenna these reports are about the same or slightly higher, however I am getting a fair bit more noise from the wire.
More testing will be done through the day.

Whilst I appreciate comparing with a wire it like trying to compare an apple with an orange, however it's all I have here to do any comparisons with. Also consider the wire is 8 meters above ground at the feedpoint and the Gainmaster is 2.8m at the feedpoint

All in all a successful project I think.
Made for less than £12 in materials and a glassfibre fishing pole that I had in the garage.
If you wish to factor in the cost of the fishing pole it was £25.
Sirio Gainmaster for less than £40 anyone? :)

Psi

Re: Homebrewing a Sirio Gainmaster using RG58 and 2.5mm wire

Posted: 06 Jan 2013, 12:26
by Windy_Miller
So the design is not rubbish after all, as some tried to make out when they first came out? ;-)

I guess like so many other projects, the economics can very somewhat if the manufacturing time is taken into account, and whether or not things you have "commonly lying about" are the same as others have too. Being self employed, if I took the time it takes to make one into consideration, it would cost more than buying one. Put another way, it would be more cost effective to be out making money, then spending some of that money on buying one which has been mass produced, faster and using less man hours than I could ever hope to achieve.

Anyway, its good to see that all the initial scorn and ridicule that was poured on this product and its design when it first arrived seems to be melting away, and I read in other threads here that they are becoming popular in some areas, and even the antenna of choice! Pretty much mirrors the ridicule that the Chinese designed and made radios got when they too first came out - Now, you can hardly look at a signature file radio listing without it containing Anytones or Baofengs!