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Letters and emails should contain complete name, address and daytime phone number. Letters to the Editor are submitted on the condition that Silicon Chip Publications Pty Ltd may edit and has the right to reproduce in electronic form and communicate these letters. This also applies to submissions to "Ask SILICON CHIP" and "Circuit Notebook".

Nissan Leaf is just too expensive

With reference to the recent mail on the subject, would you be more likely to buy a Nissan Leaf if it were $35,000 or a Mitsubishi iMIEV for $30,000?

These are the prices in the USA before their Government subsidy. Surely at these prices many more Australians would take up an electric car. Note that the price difference is not Australian tax; they are not classed as luxury cars.

Last time I looked at a map, Australia is closer to Japan than to the USA so freight is not the issue.

The only reason I can think of for the much higher prices is that they charge that much because they can. I suggest that readers ask their local Nissan and Mitsubishi dealers to justify their pricing policy. Maybe exposing the price difference and publicly shaming them is the only way to get electric car prices to be more realistic.

Arthur Davies,

Canberra, ACT.

Destructive echoes in digital TV reception

I would like to comment about an aspect of digital TV (DVB-T) reception.

I install TV antennas in Perth and specialise in difficult TV reception problems. According to various papers on DVB-T (Digital Video Broadcasting – Terrestrial), the signal structure has an in-built protection mechanism to render it immune to multipath propagation (ie, echoes) within a period known as the “guard interval”.

As I understand things, VHF transmissions in Perth currently use a guard interval of 1/16 and UHF 1/8. These equate to time intervals of about 64µs and 128µs respectively. Echoes that occur in less than 64µs/128µs should not affect reception.

Radio waves travel at a nominal 300 metres per microsecond, so echoes that travel less than 19.2km to 38.4km respectively should have no effect on reception. In general this protection mechanism seems to do a good job in many locations. However there are locations where it fails and results in serious pixellation or no reception for one or more channels. This can occur in areas of both weak and strong signal strength and for both VHF and UHF.

The echoes can come from anywhere in a 360° azimuth. The echoes are signal reflections mainly from stationary, man-made metal structures which are many and varied, eg, metal masts or towers, metal-clad buildings and reinforced concrete structures. Echoes from hilly terrain and trees have been in the minority around Perth, in my experience.

Echoes can cause frequency selective attenuation and corruption of signal. They can cause large loss of signal over a large portion of the channel bandwidth. The loss can vary from 10dB to more than 20dB for 20-30% or more of the channel bandwidth. The error rate can be rendered beyond the “cliff edge” and the signal not be decoded.

Not all echoes cause problems but some definitely do. Some cause more corruption than others and I will refer to them as destructive echoes. Using timing/distance measurements from my meter, I have determined that in many cases destructive echoes have entered via the back of the antenna and in some cases via the front. The destructive echo delays have varied anywhere from less than 2µs to about 45µs. This relates to distances that vary from less than 600m to about 13.5km. These delays are well inside the guard interval.

When destructive echoes have entered from the back half of the antenna it is because it has a poor front-to-rear ratio. In these cases, one successful method to reduce the signal corruption has been to replace the antenna with another that has a better front-to-rear ratio. This generally results in increased signal level, less loss across a large portion of the channel bandwidth and improved error rate to bring it above the “cliff edge”.

The antennas that have given best results are plane reflector types that give typical front-to-back ratios of a nominal 20dB or more and front-to-rear ratios of 14dB or more across the whole of the relevant band. The plane reflector is made of mesh to reduce wind loading. The feeds have comprised simple dipole arrays and in one VHF design the feed and directors of a Yagi.

There are other aspects and techniques that can be associated with the improvement but will not be discussed here. From my experience, destructive echoes are a major cause of signal failure in locations of difficult reception. In many cases, this can be overcome by using a suitable antenna for the particular situation.

The main purpose of this letter is to highlight that the “guard interval” does not always protect the signal from echoes and that the radiation characteristics of the antenna can play a big part in overcoming destructive echoes. These two points do not seem to get mentioned in articles or papers on the reception of DVB-T signals.

When installing antennas I use a UNAOHM Proteus T40 DVB-T field strength and analog meter.

Mike Hale,

Maylands, WA.

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