Q: Biquad Antennas

"stephen" hath wroth:

Nope. Lots of assorted additives and stabilizers for PVC and vinyl. Mostly heavy metal heat stabilizers in PVC. The white PVC pipe contains a little copper. The problem is that the exact formulation varies somewhat by manufacturer so testing is required. |

dissapation aside won't the PVC itself detune the antenna?

Oh yes. Shoving an antenna into a PVC pipe or tube[1] will cause the resonant frequency of any tuned wire lengths to drop in frequency somewhat. The good news is that it's a function of the square of the distance between the wire and the PVC. Farther away means a smaller shift. If the PVC pipe is fairly large compared to the antenna, the effect will be minimal. If I can ever get my borrowed Wiltron network analyzer to play, I should be able to both demonstrate and measure the effect. (Yet another project).

The fiberglass I build is basically like what is used on sailboats, but I cure mine sandwiched in mylar. This restricts oxygen, which for some reason makes the resin get rock hard. It's a technique used by amateur rocket makers. Obviously, I'd skip the carbon fiber.

One advantage to building your own tubes is you can adjust the diameter.

Shouldn't the full wave loop have a balanced feed?

I think you're really asking why there is no balun. The loop is a balanced antenna, but the coax is not. It's just like a dipole. If you don't want the coax to radiate, it needs a (sleeve) balun.

Driving a balanced antenna from an unbalanced feed changes the directional pattern of the antenna. Does your software model this?

snipped-for-privacy@sushi.com hath wroth:

No. If you look at the biquad model, the coax cable feed is not shown. The generator is right at the connection point between the two quad elements.

Edmund hath wroth:

Congratulations. Which plans did you use?

This one works nicely: |

Well, using Netstumbler or Kismet to measure the signal strength difference will tell you the gain. The typical biquad has between

Ok, that qualifies as an improvement.

Hmmm... so much for the improvement. Have you tried your radio outside with the stock antenna for comparison?

Oh-oh. Nothing is that easy. Photos?

Hi, I was lurking here for some time and after reading all this in addition to what I already found about biquads, I decided to make one myself. I really don't know what I am doing :-) and I wanted to make the biquad for mounting in a parabolic disk, so I made an 40 mm round mounting pipe on the back. Which fit exactly in a normal LNB holder. Yesterday I bought a cable ( 50 Ohm ) and an SMD connector which fit in my PCI wireless card. The other end I soldered directly on the antenna. I decided to test without a parabol and guess what? I see about 15 networks and after some pointing a can connect and have a good and rather fast internet connection. I have no idea where this router is and no idea about any gain I have with my antenna but the standard antenna sees 0 networks. Bud of course this is indoors and the biquad is outside, with about 3 meter cable. I am not sure if it is alright what I have done bud maybe it is of any use for someone else. I took a 8 mm bold and drilled a hole through it of 4.5 mm. I used this to support my biquad and the stripped cable connects OK in the 4.5 mm hole. I soldered one side of the biquad on top op this bold, and after the cable was pushed trough a only soldered to center of the cable to the other end of the biquad. Really easy.

For the moment I use some 40 mm PVC fittings to support and aim my biquad, which is easy too since I have a 40 mm PVC pipe fitted on the back of the biquad already :-) I am a little surprised It went all so smooth.

On Thu, 20 Jul 2006 22:04:07 +0000, Edmund wrote in :

Use

Well I didn't exactly copy any plan, I just picked what I needed and came up with some idea's of my own. I did look good to travormarshall.com and my reflector also has 30mm lips and is 110 X 123 mm.

Not possible, it is a bigtower there is no space behind the computer for the biquad either. In fact, that was the first reason I needed an external antenna. What I can do is connecting the biquad inside and see what network it sees inside, I don't expect much however.

I just took a few photo's, they show some nice detail, maybe you can give some comment about it. I don't have a home page for this stuff, shall I mail it to you? Feel free to put it on line if it of use for anybody.

Maybe one of those "coax" baluns would do the trick. The most harm it could do is the loss in the cable.

It's funny that the gain direction of the short loop is opposite (more accurately perpendicular) to the full wave loop. I've done HF DXing with a short loop, and the null is amazing. I have need for DFing some UHF signals, so I'm going to try a full wave loop.

One of the things to keep in mind is most of these home made antennas have larger aperture that stock antennas. So even if you don't achieve the ideal gain, just that fact the "sponge" is larger should deliver a stronger signal to the wifi card.

My favorite VHF/UHF DF antenna is any pair of omni antennas, seperated by 1/4 wavelength, and connected to gether with a 1/2 wave electrical coax cable and a T connector. The phase of the direct signal to the antenna with the T connector is exactly 180 degrees phase shift from the other antenna. That results in cancellation and a really deep null. One catch is that the notch depth is really frequency sensitive.

The original Trevor Marshall photos do not show much detail on how the feed is built. Look at some of the others posted to get an idea of what's necesary.

Sure. Email to snipped-for-privacy@cruzio.com. I can post it on my web pile if it looks interesting.

Yep, but not quite a sponge. The reason that many home made antennas appear to work is that almost any type of larger antenna is an improvement over the stock rubber ducky antennas. Even the most disgustingly mis-cut antenna will have some gain that is greater than the stock antennas 2dBi. Even when built correctly, the typical coffee can or biquad are sufficiently broadband that errors in cut lengths will not have a huge effect on the gain.

What I do is compare the antenna with a known gain reference antenna. I can't measure the actual gain directly, but I can measure the gain relative to my standard.

I agree the gain is higher, but I don't think you can ignore the aperture either. If the field is uV/meter and your antenna is larger (i.e. sits in more of the field), you will "soak" in more signal. Hence my reference to the sponge.

The aperture effect is more evident as you compare the same antenna design (i.e. same gain) over different frequencies. If the antenna designs are the same, the lower frequency antenna has a stronger signal since it has more aperture. [Obviously, these tests are done at the appropriate frequencies.]

I did look around and used about the same method as here:

As you will see on the photo I shortened this end cap to have a little more slack for positioning the bequad in the dish mount.

Maybe you can show it anyway, as how it is not to be done :-)