Archive for February, 2010

Interesting tunable TV antenna (2)

Wednesday, February 24th, 2010

This is probably one of the most interesting but impractical antennas I’ve come across. It might have been very useful in the days when TV had only one or two channels but how on earth would men be able to channel hop when each time you first have to get up and tune your antenna? Maybe in those days they did’?t know how to design wideband UHF antennas with > 6 dB gain?

However, it is still a very fascinating idea – having a tunable antenna, so after some investigation I think I might have figured out how it works.

I made a model and simulated the antenna with different tuning positions and the resonant frequency increased as I increased the tuning stub (similar to pulling it upward) as shown in the graph below.

S11 vs Frequency for different tuning lengths. The frequency increased as the antenna arms were shortened.

S11 vs Frequency for different tuning lengths. The frequency increased as the antenna arms were shortened.

Radiation pattern of the antenna tuned to 600 MHz with maximum gain of 6.2 dBi.

Radiation pattern of the antenna tuned to 600 MHz with maximum gain of 6.2 dBi.

The gain of the antenna is +- 6 dB as shown in the image above. This is quite high for a folded dipole but it seems like a very clever cross between a folded dipole and a V-dipole. Another interesting observation was that almost no currents flowed on the tuning stub which means it has almost no effect on radiation.

The length of the loop is about 3 lambda at the resonant frequency thus instead of using a standard folded dipole with one wave length circumference it operates in a different resonant mode with a V-dipole-like angle making it more directive.

If I had no choice but to use this antenna I would have probably built a remote control stepping motor to tune the antenna to the right frequency on each channel hop. Maybe I must still do this and invite a couple of friends over to come watch TV and channel hop at my house!

Read the previous interesting tunable TV antenna post here.

Author: Robert Kellerman

Interesting tunable TV antenna

Wednesday, February 17th, 2010
Interesting tunable TV antenna

Interesting tunable TV antenna

One of the engineers found this antenna lying in his garage. We had a quick chat about how it radiates and it turned out to be quite an interesting discussion. Someone said it’s a loop antenna, others said it’s a folded dipole. Another guy said the manufacturers probably didn’t have a clue how the thing works. They just took a bunny ear and told the manufacturers to make it tunable. So I thought lets blog about it and see what you (our readers) think.

The antenna is fed at the base as showed below.

Feed of the tunable TV antenna

Feed of the tunable TV antenna

TV antenna being tuned

TV antenna being tuned

The image above shows how the tuning part pulls in and out which seems like it’s a short circuited transmission line that varies the admittance while changing the resonant length of the antenna. Below is an equivalent circuit diagram of the antenna.

Equivalent circuit diagram.

Equivalent circuit diagram.

Please comment if you think you have a better idea how it works.

Author: Robert Kellerman

SKA – single element overview

Thursday, February 11th, 2010
Single Meerkat radio telescope element

Single Meerkat radio telescope element

The image above shows a single telescope forming part of the Meerkat project which is planned to form part of the future +- 500 telescope Square Kilometer Array.

The feed consists of two dual-polarised horn antennas, five conical deflector cones (where future horns could be inserted), cooling components, LNA’s and RF- to-fibre modulators. The struts are also designed to maintain strict tolerances on the feed position relative to the parabolic reflecting surface.

The 12m dish is made of a resin-foam composite and has a parabolic shape with 0.38 F/D ratio. It has to maintain its shape within millimeter tolerances and remain an almost perfect electromagnetic reflector while withstanding harsh environmental conditions like temperature variations, rain and strong winds etc.

The dish mount is an altitude-azimuth system and rests on a concrete pedestal. Another challenge is constantly stabilising the telescope, compensating for external forces like wind, gravity or metal expansion (and contraction due to large temperature changes) which can pull the whole structure off target or cause unwanted spillover.

In the next few blog entries I will discuss these interesting and potentially challenging topics in a bit more detail.

Author: Robert Kellerman

Egg beater antenna in 160km/h winds below -50 °C!

Tuesday, February 2nd, 2010
Egg beater antenna.

Egg beater antenna.

I recently heard that this egg-beater antenna was considered for installation at South Africa’s base in Antarctica, SANAE. It had to survive extreme weather conditions such as 160km/h winds and temperatures below -50 °C.

The egg-beater antenna is ideal for operating in UHF and VHF bands and it’s a sturdy antenna capable of withstanding extreme conditions like these. In 1983 they measured the coldest temperature ever recorded on earth: -89.2 °C at the Vostok station in Antarctica! One of my former work colleagues is currently a senior engineer on the SANAE team and he is coming to visit in a couple of weeks and I’m looking forward to interview him and hope to post a blog with a couple of his photos and stories.

Antarctica SANAE base.

SANAE base in Antarctica.

Antarctica SANAE base mountain.

Mountain near SANAE base.

Author: Robert Kellerman