Antenna question.

[allcart]

Ok so I have looked at the DIY antenna described in the Sky at Night  article and it looks fairly simple to build. I have also looked at a few other option that are ready made and not very expensive.

My question is, I have seen a few antenna's which specify different amounts of gain. So how relevant is the gain number when using an antenna for receiving only. Does higher antenna gain make it more sensitive? Give it more range?

[Tinker1947]

This may help....

http://www.antenna-theory.com/basics/gain.php

[The Admiral]

The gain is just as relevant to receiving as transmitting. Any gain you get that is noise free must be a benefit I'd have thought; better to have a larger signal input than trying to increase gain electronically. Though with strong signals, that isn't necessarily what you want. Remember, though, that gain is achieved by making the antenna more directional. That is, more sensitive in one direction than another. Directionality is useful because it allows you to reduce unnecessary and potentially interfering signals in favour of the ones that you are seeking. The downsides of this are that you need more elements to get more directionality, and that makes the antenna physically larger and unwieldy, and you don't want the directionality to be so sharp that you will not be covering the span of sky required (if you are looking for meteors for example). I've used a 4-element Yagi (similar to the S@N design) in my loft, and am now trying an LFA-Q quad antenna from Innovantennas. For info, I've attached the sensitivity plots for this antenna

You may see that this gives a gain of  9.45dBi in the forward direction (when in "free space", which it won't be of course), and a front/back ratio of about 32.5dBi. In other words, the antenna's sensitivity is predominantly in the forward direction. It has a "beamwidth" of about 60° (i.e. most of its forward sensitivity comes from the 60° in front of it). In the vertical plane the antenna shows a number of deeply cut lobes (a function of being close to the ground as opposed to being in free field, rather than inherent in the design), and the lowest one at 2.9° above the horizon gives a gain of 15.3dBi.

As to range, two things to note. Seeing a signal depends on being able to distinguish it from noise and interference. If it's a weak signal, then that becomes more and more difficult. Hence the need for antenna gain and directionality. But assuming that you can detect the signals, there will still be a limit to how far you can see. To a first approximation I'd say line of sight, give or take a bit. But it is surprising just how far that gets you when you are considering meteor scattering at ~ 100km above the ground. After all, you can still see the ISS when it is over the Med! (But then that is at 400km altitude!)

Hope I've not sent you to sleep with all this!

Ian

[The Admiral]

This link is also useful.

http://www.radio-electronics.com/info/antennas/

[allcart]

Thanks for the links guys, you can never have too much knowledge and knowledge is the key to understanding.

No I didn't fall asleep reading that Ian, it all helps.

I have kind of narrowed it down to a 5 options.

#1. Build a Yagi in 3 or 4 element.

 #2. http://www.moonraker.eu/amateur-radio/beam-and-yagi-antennas/hb9cv/hb9-2-2-element-2-metre-beam-antenna

#3http://www.ebay.co.uk/itm/2el-144MHz-2m-Quad-Style-Yagi-Shorter-than-HB9CV-just-14cms-long-HUGE-GAIN-/290871701262?hash=item43b94def0e:g:ONcAAMXQjWtRMGeI

#4. http://www.ebay.co.uk/itm/290812885021?_trksid=p2060778.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT

#5. http://www.ebay.co.uk/itm/4el-144MHz-OWL-2m-Yagi-Rear-Mount-Antenna-ideal-for-Portable-use-/290813010128?hash=item43b5ce60d0:m:muBzXV4pLghpnHOztbumQxg

As this antenna will be in the loft space which is not huge, size is a consideration too. All these options are affordable and small enough for my purpose.

But which one??????

ps. having read your post again Ian, it seems the 4 element Yagi is what you were using and the Quad is what you are now using. opinions on these would be welcome too.

[The Admiral]

Allan

First of all, let me say that although the S@N article recommend using the antenna in the vertical plane, personally, I don't think that there is much difference between the two orientations, horizontal or vertical. I haven't proved this by using an antenna in the two orientations, though running antenna design software, there doesn't seem to be a great difference in sensitivity plots. Some argue that the orientation of polarization is important. Not so sure on that. For practical reasons my antennas have been in the horizontal plane, and they work. Steve Nickols also uses his horizontally and he seems to get good results, even though he lives a lot further north than either of us.

The 4-element Yagi I used to use was not based on the S@N design, though it was a Yagi, but it was something I got off the web. There seem to be a number of permutations of element lengths/spacings kicking around. The one I'm now trying is a 3-element version of your #3. I got it direct from Innovantennas. However, a word of warning; the design has changed a bit and the picture isn't truly representative, the one I received is narrower and taller. Here's a pic of it.



It's about 50cm per side, and probably looks bigger in the pic than it actually is. Nevertheless, I found this a bit irritating because I was less able to tuck the antenna under the sloping roof, and also it was such a tight squeeze getting it through the loft hatch that I had to partially disassemble it. Needless to say, the less work I have to do in the loft itself the better. I haven't had my Quad antenna for long, but I think it has a slightly lower noise level than my Yagi, and certainly appears no less sensitive. I haven't had as much sporadic interference with it either, so far. All very subjective though, because background noise levels seem to vary, although checking immediately before and after swapping did show a reduction. I'm satisfied with it and I've no plans to swap back. Steve's set-up produces far lower noise levels than mine, though he has his S@N antenna up a post in the garden.

Loft spaces, it has to be said, are not ideal places to put antennas, but needs must. You get attenuation from the building structure, which also plays havoc with the antenna characteristics, and there's a lot of electrical noise. The up-side is that it is good and high, and that's important as you need a good view down as close to the horizon as you can get. The reason I opted for a 4-element Yagi initially was I thought it might offset to some degree the attenuation of the roof tiles. I wouldn't entertain a 2-element antenna in the loft. None of this is terribly critical though. The 4-element OWL (#5) looks a reasonable choice. In fact, IIRC someone here uses a 3-element externally mounted OWL.

Whatever you choose, make sure that you use a good quality low-loss co-ax suitable for VHF and the length of cable run, and decent connectors. Westlakes is a good source (http://www.whwestlake.co.uk/ ), and they have a co-ax loss calculator on their site. I'd also recommend that you follow the simple instructions for a "choke balun" given here. It's not strictly necessary, but because co-ax is "unbalanced" and the antenna really requires a "balanced" feed, it helps prevent stray pick-up from the cable.

Hope that helps.

Ian
 

[allcart]

Thanks Ian. Shame they changed the design of the Quad. I would have to dismantle it too, and it would be more awkward to position. I will probably go for the 3 or 4 element Yagi as positioning will be much simpler. I have loads of coax and baluns and have ordered the FCD. I'll order the antenna next week and get it set up.

Will prob have a few questions about setting up the software when the time comes.

[The Admiral]

Ah, if you've loads of co-ax and baluns then I've probably been teaching you to suck eggs .  Apologies!

The FCD seems to work well, though it's got a very small input socket. I've fitted an SMA adapter lead. What software? The infamous SpectrumLab? Oh well, I wondered when that might rear its head!

Ian

[allcart]

I have dabbled in video astronomy and I have a video meteor detection system running so have used coax and baluns with those.

Not sure what software is best for radio meteor stuff yet, although SpectrumLab does seem to be well used.

[The Admiral]

For what it's worth I started off with SDRConsole v2 (http://v2.sdr-radio.com/ ), as I wasn't happy with the installation of SDR#. I see v3 is now out. Quite useful just for listening around the bands as well as meteor detection. I eventually became fed up with sitting in front of the PC waiting for a meteor strike, so ended up using SpectrumLab. Quite a different kettle of fish with quite a steep learning curve. The one advantage is that you can run a script to configure it and get it to do what you want, including identifying events, recording them, taking screen-shots of them, counting them, etc. Prepared scripts are available, which is as well as I found writing them to be a challenge!

See http://www.rtl-sdr.com/big-list-rtl-sdr-supported-software/ for a listing.

Ian

[allcart]

Thanks Ian, I'll look into software when I get set up. SpectrumLab does seem to be the way to go. I like automation.

[allcart]

Finally got the antenna. Question. It comes ready to assemble and has mounting blocks to secure the elements to the boom. The blocks come in two parts, 1 plastic and 1 aluminium. Does the plastic block go next to the boom to insulate the elements and then the ali block on top to secure it.? 

Or should the elements be in contact with the boom.?

[The Admiral]

Allan, that's a new one on me! Is it an Innovantenna make? If it is you should either have a leaflet in the box (which if you are lucky might actually refer to your particlar design!), or else they should have emailed you one. Drop them an email and ask the question, they are very responsive, even over weekends I found. With my antenna the elements went through a hole in the boom and were bolted through.

If not, my guess would be that the elements should be electrically connected to the boom, though not the driven element of course as that would short the connection. Does the driven element have a polythene insulator in the middle, or is it a folded element with free ends?

Ian

[The Admiral]

Allan, if it is an Innovantennas 3-element LFA Yagi, I spotted this on their website:

"This antenna is made with a 1/2 inch (12.7mm) and 3/8 inch (9.525mm) diameter tube LFA loop and 1/4 inch (6.35mm) solid rod elements. It also has fully insulated elements which will ensure continuous, high performance for many years to come. Boom is 1.25 inch square 16SWG aluminum." (My emphasis).

So perhaps the plastic block does go between the boom and the element. It's a pity they don't have a picture on their website. Still, worth an email to them as they should have provided you with assembly instructions.

Ian

[allcart]

Hi Ian, yes that is the one. I didn't spot that quote. I've looked at various pics and could not get a satisfactory answer. I will email them just to check.

[The Admiral]

Allan, have a look here. It's an article by G0KSC, the originator and owner of Innovantennas. It'll give you some idea of what the antenna is about.

Ian

[allcart]

Thanks Ian, lots of interesting info there.

[The Admiral]

It does occur to me that even if the bottom half of the element clamp is plastic, the element will still be connected to the boom through the metal top half and the metal bolts. Perhaps all will become clear in a response from Innovantennas. Perhaps also you shouln't assume that you have all the bits :-). I was missing both the assembly instructions and a mast clamp! All well in the end though.

Ian

[allcart]

From Justin at Innovantennas;

"The metal clamp (that is the same as the green ones) fits on the loop opposite the feed point, the green clamps are on all others."

He also sent me the instruction manual.  Hopefully all the bits are here.

[The Admiral]

Thanks for the info Allan, you should be able to make progress now. I presume that the "green" ones are the insulating mounts.

Ian

[allcart]

Yes, Its up in the loft waiting to be assembled, but IIRC there is one metal block and lots of plastic blocks. It should be relatively simple now. Next problem will be running the coax out of the loft and back in to the dining room obsy.