Page 2 of 3

Re: Moonraker antenna?

Posted: Mon Dec 02, 2019 10:33 pm
by Albert H
livendirect wrote:
Mon Dec 02, 2019 9:35 pm
i have ordered one to compare with a dipole going to use same pole and coax see what the difference is
I've done that already. The field strength differences suggest about 3.5dBd gain when the length of the radiating elements is carefully trimmed to suit the frequency.

Re: Moonraker antenna?

Posted: Mon Dec 02, 2019 11:03 pm
by Bton-FM
Does anyone have a link to build one instead of buying?

Re: Moonraker antenna?

Posted: Mon Dec 02, 2019 11:08 pm
by Albert H
A similar design - that works well - is the "Super-J". You have to scale the dimensions from the 2m version. It's not an easy aerial to build, but if you get it right, the performance is amazing.

Re: Moonraker antenna?

Posted: Mon Dec 02, 2019 11:11 pm
by Bton-FM
Don’t they fall over from wind?

Re: Moonraker antenna?

Posted: Mon Dec 02, 2019 11:14 pm
by Albert H
http://sm0vpo.altervista.org/antennas/6dbvhf0.htm

Harry's a bit optimistic with his gain figures, but it's a brilliant aerial. I've used these for years.

Re: Moonraker antenna?

Posted: Mon Dec 02, 2019 11:16 pm
by Albert H
Bton-FM wrote:
Mon Dec 02, 2019 11:11 pm
Don’t they fall over from wind?
Not if you build them properly. I used to join the two radiating elements with a tightly-fitting nylon rod inside.

Re: Moonraker antenna?

Posted: Mon Dec 02, 2019 11:38 pm
by Bton-FM
Does the colinear perform better than 5/8 vertical?

Re: Moonraker antenna?

Posted: Tue Dec 03, 2019 5:48 pm
by NOYB
Anyone know a UK supplier of the Comet 5/8 antenna?

Re: Moonraker antenna?

Posted: Wed Dec 04, 2019 1:07 am
by Albert H
Bton-FM wrote:
Mon Dec 02, 2019 11:38 pm
Does the colinear perform better than 5/8 vertical?
Yes - significantly. The ⅝-wave is (usually) a bugger to match accurately, but Comet seem to have done some magic with their base loading coil to broaden out the bandwidth. Unfortunately, wider bandwidth always comes at the cost of reduced gain.....

As in almost all engineering, antenna design is a set of compromises. The http://sm0vpo.altervista.org/antennas/6dbvhf0.htm is in effect a single slot colinear. It's easy to build, and my original ones - made with some H-section aluminium rods used (originally) for room dividers - really went well. My "puny" little 40 Watts was easily equivalent to some people's 100W into a dipole!

The only problems that I encountered were making good connections to the aluminium rods - I sorted this out by using self-tapping screws. I first determined the exact feed points using crocodile clips, marked the locations and drilled small pilot holes to start the self-tappers. I soldered the coax connections to the self-tapping screws. I also found that I had to seal up the ends of the coax that were exposed to the rain, otherwise water would creep up inside the coax causing all sorts of problems! Once all the feedlines and the phasing loop were made and tested, I sealed all the ends with epoxy glue. I left the thing in my garage to dry overnight - the stink from the Araldite was horrible!

The first one was installed on a fairly low site, with a low power rig (40 Watts) (described as nietig!) and I was pleasantly surprised by the coverage it achieved. I passed the plans to a friend in London, who made a similar aerial for "Quest FM", and got great results at fairly low power.

I've built Super-J aerials and got really good results. The secret is not to have the phasing stub bear any of the weight of the upper section. I achieved this by using a rigid nylon rod that fitted tightly inside the upper and lower vertical radiators. I got the idea from a J-Beam design from some years before. You can also use a further nylon pipe to enclose the weakened "join" section, since the nylon will have no significant effect on the radiation. I've also built colinears inside fibreglass pipes - a great way of keeping everything dry!

Re: Moonraker antenna?

Posted: Wed Dec 04, 2019 8:45 am
by RF-Head
The moonraker is a normal 5/8 with a coil in the bottem not more nothing less
the design is almost the same as the comet only with fiberglass over the element(thin brass)

Re: Moonraker antenna?

Posted: Wed Dec 04, 2019 8:48 am
by teckniqs
Albert H wrote:
Wed Dec 04, 2019 1:07 am
I passed the plans to a friend in London, who made a similar aerial for "Quest FM", and got great results at fairly low power.
Picked that station up a couple of times around late '93, if it's the same one...

Re: Moonraker antenna?

Posted: Wed Dec 04, 2019 9:06 am
by ronald001
I personally use a "Norwalk Dominator" which is a remake of the Sirio Vector 4000 for FM Broadcast. ( PCS electronics has one aswell, and named it the "exterminator")
Some say that the performance equals to a pair of stacked dipoles.
I never had 2 dipoles - but what i do know is that it's way - way better than a single dipole.
Specs are 5.15Dbi - 3Db

Re: Moonraker antenna?

Posted: Wed Dec 04, 2019 9:10 am
by teckniqs
ronald001 wrote:
Wed Dec 04, 2019 9:06 am
I personally use a "Norwalk Dominator" which is a remake of the Sirio Vector 4000 for FM Broadcast.
And the 'Sigma IV'

Re: Moonraker antenna?

Posted: Wed Dec 04, 2019 9:39 am
by ronald001
RF-Head wrote:
Wed Dec 04, 2019 8:45 am
The moonraker is a normal 5/8 with a coil in the bottem not more nothing less
the design is almost the same as the comet only with fiberglass over the element(thin brass)
Does the comet has a LC matching circuit?

From what i have seen is that the radiator is connected directly to the SO plug ..

Re: Moonraker antenna?

Posted: Wed Dec 04, 2019 12:44 pm
by Albert H
The one I had here had a base loading coil of about 4cm diameter, about 10cm long. There may have been capacitors involved in the base assembly - I didn't take it apart to find out. It wasn't particularly effective - but it didn't look like a dipole!

Re: Moonraker antenna?

Posted: Wed Dec 04, 2019 10:23 pm
by Bton-FM
is this right for the collinear?excuse the 5 year old ms paint skills:

Re: Moonraker antenna?

Posted: Wed Dec 04, 2019 10:54 pm
by Bton-FM
Attached the wrong image but the question it was illustrating was are the two vertical elements half wave and the the distance between the feedpoint and the bend for the vertical elements 1/4 wave?

Re: Moonraker antenna?

Posted: Wed Dec 04, 2019 11:12 pm
by Bton-FM
??

Re: Moonraker antenna?

Posted: Thu Dec 05, 2019 12:18 am
by teckniqs
Sorry but what is that supposed to be?? !

....it looks like some kind of Dildo

Re: Moonraker antenna?

Posted: Thu Dec 05, 2019 2:01 am
by Albert H
http://sm0vpo.altervista.org/antennas/6dbvhf0.htm shows you how to do it. His dimensions are for 145 MHz. Recalculating for ~100MHz is easy!