2.4 GHz link - best performance with one antenna H and one V! WTF? - Page 1
Forum section: electronics / rf-microwave

author: Delta
author_rank: Super Contributor
timestamp: January 12, 2016, 01:48:00 pm
content: I've got a cheapo analogue (PAL) CCTV camera in a shed, and I'm sending the signal to the house using a pair of cheapo 2.4GHz (analogue) A/V Tx/Rx boxes (not these actual ones, but you get the idea -http://www.accessories-shops.com/images/pdimg/WVT810BD.jpgThe Tx is against a window, with the standard antenna.The Rx is in the loft, with a big Ebay Yagi on it.Distance about 20m. The internal roof is boarded and the roof tiles are quite hefty.Initially I set it up with horizontal polarisation (ie, the standard aerial pointing sideways, parallel to the house) and the Yagi also horizontal (elements parallel to the floor), my thinking being that most WifI routers use vertical pol.It worked OK, but I thought I'd try V, so I first moved the Rx Yagi to V, I notice the picture immediately got much better! Then moving the Tx ant to V (pointing it to the sky) the picture degraded!I've left it with Rx V and Tx H, and whilst I'm happy with the picture, the OCD side of me says this is not right! What is going on?

author: AF6LJ
author_rank: Unknown
timestamp: January 12, 2016, 01:58:15 pm
content: Surrounding materials will have an effect on your antennas performance.While you say you are using a yagi for your RX is it a yagi made up of dipoles or loops??

author: Delta
author_rank: Super Contributor
timestamp: January 12, 2016, 02:04:36 pm
content: Quote from: AF6LJ on January 12, 2016, 01:58:15 pmIt's this kind of affair...http://en.arronna.com/imageRepository/850f6cf1-40aa-405f-b97c-83776bf328e3.JPG

author: PA0PBZ
author_rank: Super Contributor
timestamp: January 12, 2016, 02:08:03 pm
content: The polarization probably changes due to reflections and such. Stop worrying and enjoy.

author: AF6LJ
author_rank: Unknown
timestamp: January 12, 2016, 02:26:51 pm
content: Quote from: Delta on January 12, 2016, 02:04:36 pmOkay Dipoles...I think the above post has the actual answer, not knowing your installation my best guess also would be the polarization has flipped due to reflections.

author: Delta
author_rank: Super Contributor
timestamp: January 12, 2016, 02:43:54 pm
content: Ah, that would explain it then!There is a "plane" of about 2.5m of wire mesh directly in front of the Tx antenna!PS. I'm wasn't worried, just confused!

author: AlessandroAU
author_rank: Regular Contributor
timestamp: January 12, 2016, 02:45:08 pm
content: Quote from: Delta on January 12, 2016, 02:04:36 pmThat antenna is less than useless. Some antennas of that style from china have more gain when facing the wrong way. Throw it away and make your own.

author: Delta
author_rank: Super Contributor
timestamp: January 12, 2016, 03:05:03 pm
content: I wouldn't say less than useless, as it does at least work. It seems to have some directivity, as I needed to be quite accurate when aiming it...

author: AlessandroAU
author_rank: Regular Contributor
timestamp: January 13, 2016, 08:45:09 am
content: Quote from: Delta on January 12, 2016, 03:05:03 pmThat may have been harsh. I'm a little bitter about these having wasted quite a bit of time trying to get one to work.Have a read herehttps://asdfghjkl.me.uk/blog/antennas/

author: Delta
author_rank: Super Contributor
timestamp: January 13, 2016, 12:36:18 pm
content: Quote from: AlessandroAU on January 13, 2016, 08:45:09 amErm, were you using a 2.4GHz Yagi for 2.1GHz 3G.....?

author: PA0PBZ
author_rank: Super Contributor
timestamp: January 13, 2016, 01:20:47 pm
content: Quote from: Delta on January 13, 2016, 12:36:18 pmEdit: removed picture of antenna showing 2GHz band, site no longer wants to show it...

author: AlessandroAU
author_rank: Regular Contributor
timestamp: January 13, 2016, 01:27:34 pm
content: Of course not, the point was to demonstrate the horrible construction quality. Regardless, if you read the site I linked you will see that person examines the common 2.4ghz antennae as well.Personally, I got not better performance on my eBay yagi than the standard 5dbi dipoles I was trying to improve on.

author: Delta
author_rank: Super Contributor
timestamp: January 13, 2016, 02:53:13 pm
content: Sorry, my mistake, I zoomed in on one of the photos that said 2.4GHz, it must have only been an example photo...

author: Delta
author_rank: Super Contributor
timestamp: January 13, 2016, 04:28:48 pm
content: Quote from: AlessandroAU on January 12, 2016, 02:45:08 pmRight, I owe you an apology Alessandro; you were spot on, the eBay Yagi is in enormous pile of shite.It is indeed worse than useless.I've been playing about whilst looking at the demodulated video signal with my scope (I don't know what that says about me: I don't have a TV I can take up to the loft, but I do have a scope!) and it was an utter pile of pish - the telly was doing a pretty good job getting a picture out of that!I opened the grey box, and there's an attempt at a 1/4 stub BalUn, but shouldn't that be shorted at then end? (It wasn't!) I ripped that out, and have now got a decent signal with the "Yagi" pointing in a completely random direction, up and away from the Tx.So I will now take your advice, chuck it away, keep the SMA tail, and make my own.Any tips please Sir?

author: PA0PBZ
author_rank: Super Contributor
timestamp: January 13, 2016, 06:14:48 pm
content: Quote from: Delta on January 13, 2016, 04:28:48 pmA balun is like it says to convert from balanced (the dipole) to unbalanced (the coax). If you take that out you will completely destroy the radiation pattern because the coax now is part of the dipole. So your observation of random direction is correct.

author: Delta
author_rank: Super Contributor
timestamp: January 13, 2016, 06:22:57 pm
content: It was already random directioning before I touched it!But should the stub balun be shorted for it to work properly?

author: PA0PBZ
author_rank: Super Contributor
timestamp: January 13, 2016, 07:22:15 pm
content: If it is a folded dipole the impedance will be about 300 Ohm, so you will need a 4:1 balun to connect it to 75 Ohm coax, like this:If you want to calculate the length go here:http://www.n-lemma.com/calcs/dipole/balun.htmFind out the velocity factor of the coax used, or assume it's 0.66 if you don't know the type.

author: AlessandroAU
author_rank: Regular Contributor
timestamp: January 13, 2016, 08:44:53 pm
content: Quote from: AlessandroAU on January 13, 2016, 01:27:34 pmEdit:WordingQuote from: Delta on January 13, 2016, 04:28:48 pmI'm a really a big fan of circularly polarised antennas for analogue video because it helps with multipath issues! I would give a helical antenna a try:http://www.w1ghz.org/antbook/conf/Helical_feed_antennas.pdfyou can find alot of designs on the internet. I am also a fan of the cloverleaf antenna if you're after a omni circularly polarised antenna.http://www.rcgroups.com/forums/showthread.php?t=1388264

author: AlessandroAU
author_rank: Regular Contributor
timestamp: January 13, 2016, 09:03:05 pm
content: Quote from: AlessandroAU on January 13, 2016, 08:44:53 pmBiquad antennas also work pretty well and have a small form factorhttp://martybugs.net/wireless/biquad/I have used that design on a router and it had excellent performance.By the way, have you tired something as simple as a parabolic reflector behind the standard dipole antenna? Something like this:http://www.freeantennas.com/projects/template2/

author: Delta
author_rank: Super Contributor
timestamp: January 13, 2016, 11:17:43 pm
content: Quote from: AlessandroAU on January 13, 2016, 09:03:05 pmWell much to the Missus's confusion, I got out the scissors and glue, and made a windsurfer...Fecking feck me with a fecking barge pole, the picture is now PERFECT!The engineer in me thought "well, it's a metallic parabolic antenna, this should work", but the bloke in me said "don't be daft, how can something you've made out of card and foil possibly do anything?"Wow, very impressed, thank you Alessandro!

author: Delta
author_rank: Super Contributor
timestamp: January 14, 2016, 12:51:56 am
content: I think I've got the radio bug now, so I've going to have a go at making a Biquad tomorrow. Are the dimensions of the reflector critical? I've got a few bits of plate kicking around but they are bigger than the 123x123mm (one wavelength) given in the link, and I've seen pictures of them made with all manner of reflectors...

author: AlessandroAU
author_rank: Regular Contributor
timestamp: January 14, 2016, 01:32:16 pm
content: Quote from: Delta on January 13, 2016, 11:17:43 pmAwesome! haha, isn't it great when some silly simple method works so well! I made a fancier one out of wood and a soda can for the router at my parent's house. Its a pretty easy way to add some directionality.Quote from: Delta on January 14, 2016, 12:51:56 amI don't think so, I built one with a 120mm x 120mm PCB and it worked quite well. The most important dimension is the distance between the bi-quad element and the reflector which is 15-16mm depending on the thickness of the element.My favourite design that the moment is this 3D printed yagi. The elements are floral pins from a craft shop that have been cut to length and its driven by the existing dipole. The yagi adds about 10dBi of gain to the dipole giving about 11-12 total! Takes minutes to build (if you exclude printing time).

author: Seekonk
author_rank: Super Contributor
timestamp: January 14, 2016, 06:38:03 pm
content: I have actually never used these. Providing these two pictures so you can identify it on ebay. Note the black coax stub and the brass stubs are the pickup. Wasn't able to open the loop antenna.