Circular Waveguide Antenna

Tests on first construction

Our first waveguide antenna had a diameter of 96 mm, with a length greater than 3/4L_g and was made from a can which came with a bottle of gin :-)

The antenna was fitted with a standard television 75 Ohm connector (not a proper N connector). The pigtail used was made by removing the lead from a Buffalo Extended Range Antenna and attaching a standard television 75 Ohm connector. We are aware that the combination of 50 Ohm coaxial cable and 75 Ohm connectors is not a good impedance match and would result in loss, but at the time of testing, in Portugal, that is all we had, and we were keen.

Comparison was made with respect to the internal antenna on a Buffalo PCMCIA 802.11b card. Using Wavemon (a wireless measurements tool) on a GNU/Linux laptop to measure received signal strength, noise and signal to noise ratio.

Results

Our first can, including cable loss we received around +4 to +5 dB (deciBels) improvement in received signal strength, and a +10 dB improvement in signal/noise ratio.

Looking at data tables we estimate the coaxial cable used to have a 1.5dB loss.

This antenna allowed us to maintain an 11 Mbps connection at 200 metres from the Buffalo Airstation Extended Range Antenna. We could not walk far enough with clear line of sight. We were impressed that this improvement was measured even using cheap television connectors from the supermarket (at the wrong impedance).

First mountain test

Finally we managed to get hold of some 50 Ohm N-type connectors. Unfortunately no-one seemed to stock the mating pair so we had no option than to reduce to BNC and back up again at the other end. This worried us not as we were eager.

The only cable we could get hold of was RG58/U, fairly high loss

We attached a gin can waveguide antenna to the Buffalo airstation, via 10 metres of cable, and pointed the antenna out of the window towards the HILL

Iain went to the top of the HILL with his laptop, running Wavemon on GNU/Linux, and a dogfood can-tenna, via 2 metres of cable. The top of the hill has two large multi-sector mobile phone antennas, broadcasting around 800MHz, we believe. Iain was positioned roughly 50 metres below them (slowly frying his brain!).

We had a clearish (tree tops) line of sight down the valley which measured 2200 metres point to point on the military map.

Results

We achieved a 2Mbps connection, with around 7 to 8 dB to spare, although in the excitement the figures got a little lost. Well, completely lost. The important part for us at this point was that it worked.

Having looked at the specification sheet for the cable we were using, it seems not to rate as high as 2.4GHz in the losses, the highest rating is at 1000 MHz which is 0.79dB per metre. This means that by using far too much of the wrong cable we have cost ourselves 9 to 10 dB. This is good news for our next hilltop shot with short good cables, and suggests a clear 5000 metre shot.

The can shown above has given us 16 to 17dB improvement over the antenna on the Buffalo wireless card. This is successful. We are very happy with the results of this can.

We are now awaiting some spare time and enough clear space to do further range tests. We are expecting 10,000 metres on a can to can link.

Circular waveguide antenna calculator - JavaScript

See Fig1.1 for how to use values - most of them are unnescessary and slightly confusing.

• D is the interior diameter of the can
• L_o is wavelength in open air = 0.122 metres
• L_c is wavelength at lower dominant mode cut off frequency
• L_u is wavelength at higher dominant mode cut off frequency
• L_g is standing wavelength inside can

L_c = 1.706D

L_u = 1.306D

L_g = 1 / (sqr_rt{(1/L_o)² - (1/L_c)²})

Ideally for the usual operating range of 802.11b:

• Lower cut-off frequency should be lower than 2400 MHz
• Upper cut-off should be higher than 2480 MHz

Fig 1.1 - Circular waveguide antenna showing design values, click to enlarge.

Table 1.1 - wavelengths and frequencies against diameter

See Fig1.1 for how to use values