WiFi dish mod to 3.6GHz?

Hi SEDers!

At the industrial premises where my office is, the company next door was throwing this out:

It's alleged to be a WiFi (2.54GHz) dish.

I'm more interested in finding out how much work is involved in adapting it to the local wireless internet ISP's frequency of 3.5-3.6GHz.

The innards of the item at the dish focus are shown here:

along with some approximate measurements (+/- 1mm).

The last time I dealt with any RF theory was in University (20 years ago) and I've never used it since, so I'm hoping I can get some pointers as to whether it's worth bothering with. The local price for manufactured aerials for this ISP are in the AUD 150-200 range as far as I've found so far.

So, my guesses so far: It seems like the aluminium part is a secondary reflector?

The dipole part is quite close to the theoretical value for 1/2 wavelength at 2.54 GHz (I calculate ~59mm for 1/2 wavelength).

Is it just a matter of trimming the dipole to the equivalent 1/2 wavelength for 3.6GHz (~42mm) ?

The reason I ask is that it looks like there's some sort of balun incorporated by the way both brass shims are actually soldered to the braid, but the center conductor is tied to the 'center' (approx) of the dipole. Am I close?

If so, does that mean the length of the brass 'feeders' that are at right-angles to the dipole is critical too? If so, I presume the fold in the shim should be roughly 1/4 wavelength?

Thanks in advance for any advice offered.

Chris.

I think everything you said is about right. Getting the dipole/reflector working good at exactly the new frequency may be difficult without a way to measure return loss at the new frequency. Not to say you couldn't get lucky.

One thing you didn't mention is the mesh nature of the dish reflector. As you go up in frequency it will work less and less good.

the dish itself should work fine

the dipole can easily be trimmed smaller and the exact size is not really that critical

it may be very difficult to modify the electronics to the new frequency and modulation and any other phy layer changes needed.

Mark

Hello Chris,

Yes, and it needs to be shortened as well.

There are some other factors (vicinity to plastic etc.). Just shortening it proportionately usually works. Same with the aluminum reflector.

The connection of the "hot" element side (the one fed by the center conductor) to the coax sleeve may have to be dropped.

You'll lose the holding pegs because both elements will now be shorter. You could glue in a plastic flange to screw the center of the aluminum reflector to. For the main element maybe some potting compound can hold the center. Try to avoid affixing the ends. Any load, even dielectric, can mess up the performance because they are high impedance there.

If they "threw" it into the dumpster followed by other heavy stuff the reflector might be bent which degrades performance. Also, as Rex said, the mesh size may be a bit on the coarse side for the higher frequency. But heck, even if this antenna only gets you a few dB that would be better than nothing. Considering that it didn't cost you anything.

On a side note, sometimes higher gain antennas aren't allowed by the authorities.

Regards, Joerg

That part shouldn't be a problem because I want to try it with a Wireless broadband 'modem' (for want of a better word) which is designed to work at the 3.6GHz frequency. It also has external antenna connections.

Thanks for your reply.

Chris.

Well, I salvaged it before anything else was thrown on top of it so I'm reasonably confident the reflector's OK ;-)

I was also thinking about the mesh size and I thought I'd add a layer of brass flyscreen or similar just to be on the safe side.

I'll look up the local rules, but for now it's only a temporary test on borrowed equipment.

I live on the outskirts of town where there's no wired broadband and here in Oz, the sattelite broadband (my only retail option) is quite expensive (relatively) especially two-way. I don't rate my chances success very highly as the nearest wireless broadband tower is ~15km away and I don't have a clear line-of-sight.

Thanks for your reply! It's encouraging to think that some of that

20-year-old knowledge stuck!

Chris.

Hello Chris,

Same here in California. About $80 AFAIR. DSL is $50 and web over TV cable is in the same range but only if you also take cable TV. No way we'd do that.

I don't want to spoil the broth here but at 3.5GHz and without a line of sight the chances are pretty slim. If trees are blocking the path, maybe. But if it's building probably not.

Once you have practiced RF for a while it is like riding a bike. You may get a bit wobbly but you won't forget the basics.

Regards, Joerg

Hello Chris,

Coax losses are huge at that frequency. So if you detect a faint signal you can improve things by placing the modem really close to the antenna. Or you could then try to find one of those very large dishes at a yard sale.

Regards, Joerg

Yes, I'm just now looking around at what to use to feed the signal back into the house and it's a little disturbing (both the cost and the db loss per m)!

Chris.

I'm in NW Sydney. Who are the primary users of C-Band? The only likely installation in the area I can think of that could have sat downlinks is a RAAF base but it's ~90deg to where I want to aim.

That was basically going to be my approach.

I'll check the libray first, I think, if I need to. It's not something that's likely to crop up that much in my case ;-)

Thanks for your suggestions!

Chris.

Or you could find some parabolic Chineese cookware.

--
  Keith

One thing to contemplate as you point 3.6G around the place is that it's

*awful* close to C-band satellite downlinks. I don't know where in Oz you are but you need to be mindful, as those services will be protected.

FWIW, since you're playing, you could try 'just' shortening the lengths and distances involved in the various bits of copper/Al. You might look to applying the same 'fudge-factor' that the original manufacturer's came up with when they got their lengths to be slightly off 1/4-wave, etc. The ARRL Antenna Handbook (or was it the RSGB VHF/UHF Manual?) gave a pretty reasonable run-down on working at these freq's. Both books at Tech-Books in Melbourne.

Cheers.

Ken

Hello Chris,

I'd first have the laptop in the attic or something, maybe even on the roof with you if that is safe. Once it works you could place the modem close by. CAT-5 is a whole lot cheaper than mil grade coax.

Regards, Joerg

Hello Keith,

No joke: When I was young UK ham radio operators discovered that trash can lids in London were the perfect reflector.

Regards, Joerg

Saw this on Slashdot a while back - quite a cheap solution for WiFi in any case ;-)

Cheers, Chris.

LOL! Our bins now all have square hinged lids so... but maybe if I forget the lid & line the entire bin with tin foil... I'm thinking Pringles can on steroids! ;-)

Cheers Chris.

Yep, I've got an upstairs window that's facing in the roughly the right direction so it'll definitely be a case of minimal length coax to start with...

Cheers, Chris.

Hello Chris,

It's all plastic out here now. That makes for interesting scenarios if people don't let their wood stove ashes cool down long enough. We bought an extra couple of steel cans that we can rotate but most people have just one.

Regards, Joerg

And moved closer to the dipole, by the same factor

Exactly, but I'd find a piece of braid so that you could extened that solder-soaked part of the braid such that the "open" part of the balun is also shortened by the same factor.

Essentially, just shrink the whole assembly. Everything everybody else said is spot-on. :-)

Good Luck! Rich