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TV Reception & TV Antennas

 

   

I really must get around to updating this whole web site
but for now
just local updates as time permits.

 
 Contents


 Introduction


Australian analogue TV
is transmitted over the frequency range 45 – 820 MHz in round figures.   It is divided into five bands.

Channel 0-2 3-5 5A-12 28-35 36-69 
Band VHF I VHF II VHF III UHF IV UHF V
Freq Range
(Mhz)
45-70 85-108 137-230 526-582 582-820
Source - Australian Broadcasting Authority 2003

 

Digital Transmission Ch 6 - 12 Ch 28 - 69

 


I should preface all of this by saying that all antennas we will use in our caravans will be a compromise, a compromise between size and portability verses signal gathering ability.

What is the criteria for an effective practical antenna ?

Put very simply, an effective antenna should exhibit the following characteristics.

It should exhibit gain.

When discussing reception of TV signals it is useful to use a light beam as an analogy, also, for all practical purposes, as far as passive resonant antennas are concerned, there is no difference between reception and transmission of a signal.

Now, if you wished to illuminate a distant object with a light bulb, it is obvious that you would choose a bulb with a reflector behind it.   Why, in order to concentrate the light into a narrow beam.

This has given the light source a directional characteristic and because the distant object has a greater light intensity falling on it, the equivalent of much brighter non-directional light, we can say the reflector provided a gain to the light source.

We can therefore define gain as the ratio between the light intensity at a point from the source radiating equally in all directions and the light intensity at the same point when the same light source has a reflector behind it.

With respect to antennas, gain is expressed in units called dB (decibel) or dBi.   This is a logarithmic ratio comparing a theoretical isotropic antenna to our antenna.

(An isotropic antenna is a theoretical point in free space radiating (or receiving) equally in all directions and considered to have unity gain, although just to confuse matters, often the half wave dipole is considered to be the base reference unit and assumed unity gain.   Both references are valid.   The difference is 2.15dB. )

It should be directional.

The antenna should be directional so as to maximise the signal from the direction of the source (TV transmitter) and in the case of analogue TV, reject signals from other sources containing noise and ghosting.

It should operate over the necessary frequency range.

It goes without saying that the antenna must receive a signal over the entire spectrum you wish to receive.   In other words it must have a suitable bandwidth.

All these characteristics are closely and inexorably linked, the greater the gain, the more directional the antenna, but also the narrower the bandwidth.

For the purpose of this discussion only, there are three basic types of antenna available for your RV.


 Omnidirectional antennas

This antenna, in theory, receives a signal equally well from all directions in the horizontal plane.

There is a range of styles, usually resembling a flying saucer and mostly aimed at the marine market.

They have the advantage of being compact, robust, easy to install and weatherproof with trendy high tech appearance, but they are strictly limited to strong signal locations even though they incorporate a built in amplifier.   Because of their small size (most of them contain a circular folded dipole), they are more useful for VHF III and UHF IV & V reception.

If you have one of these you can improve low signal reception by rotating it for best signal. (The reception pattern for most of them is more cardioid than omnidirectional, there being a null point in the direction of the dipole termination.)

If relatively low cost and convenience is far more important than performance, then you could consider one of these.

They may also be useful where you were overnighting in a location where you didn't want it to be obvious you were doing so.

Due to their semi-omnidirectional characteristics, ghosting is usually a big problem when receiving analogue signals, however with the advent of digital TV transmissions, these antennas will find increasing usefulness.
Digital reception is completely devoid of multi path ghosting and noise, you either have a perfect picture or nothing, with only some "pixelation" at the changeover point.
Also, another redeeming feature for these antennas is the fact that digital transmission in Australia is confined the to the upper VHF band only and UHF bands, just where these antennas work best.  (But they are still only useful in a good signal area.)

Also, remember that these antennas by nature of their construction (with very few exceptions) are only designed for horizontally polarised signals.  Yes, you can tilt them 90
o but then you have a low gain directional antenna.
Moonraker (the only one I could recommend and in a good signal area) claim dual polarisation in the UHF bands (but horizontal only in the VHF band) with their dome antenna.
There are dome antennas with a short vertical whip sprouting from the centre of the dome to provide for reception of vertically polarised signals but for this to work effectively the horizontal element and vertical whip must be combined with a correctly designed internal diplexer.
Of those I have seen, the vertical whip has been so short that it could only be effective at the high end of the UHF band, greatly limiting it usefulness in the reception of vertically polarised signals.

Very much a compromise antenna but depending on your location and the availability of digital signals, it may suit your needs

There is more on digital TV further down the page.

Generally, I am not a fan of these antennas and would advise caution to avoid disappointment.
More drivel has been spoken and more pseudo science and ridicules claims have been made for some of these "miracle" antennas than for any other electric/electronic product I can think of.


 Half wave dipole antennas

This is a simple type of antenna that includes the 'rabbit ears'. The length of the antenna determines its suitability for receiving a particular frequency (TV channel) known as its' resonant frequency.

This type ranges from the commonly available grey or white PVC conduit 'T' shape type, with and without an amplifier, to the Winegard wind up-down-rotate model which lays flat on the roof and winds up to 1 meter above the roof.

Performance is improved on the omnidirectional type, there being a directional characteristic with corresponding increase in effective gain.   (A half wave dipole receives a signal broadside to its length and a deep null at the ends.  Because it receives a signal equally well from directly ahead and directly behind, it is said to have a 1:1 front to back ratio.)

However, it is still a long way from being a 'fringe area' antenna.
To give you a rough idea, it would need to be approximately 3100mm tip to tip for best reception of channel 0 and approximately 175mm for channel 69.

You can
calculate this for yourself with the following formula (wave length is denoted by the lower case Greek letter Lambda λ).
 λ/2 = ( 142650 / fMhz ) mm, or, Frequency (Mhz) approx  = 142650 / tip to tip length (mm).
In practice, the physical length is approximately 90% of the theoretical calculation.
For example, if your half wave dipole antenna is 1100mm tip to tip, then its' optimum reception will be approximately 130 Mhz or in the region of channel 5A.   Its' performance on other channels will gradually diminish above and below this.


 Multi element beam antennas

The Yagi

This antenna consists of a dipole (called the driven element) and a number of other elements (called parasitic elements).

The number of parasitic elements (known as a reflector if it is behind the driven element and a director if it is in front) will be chosen by the designer to give the required compromise between gain, bandwidth, size and cost for a certain application.

The size of the antenna, both element length and beam length is governed by the frequency of operation and the required gain.

To increase the bandwidth and retain the gain, there may be two or more driven elements, each a different length, that is, tuned to a different frequency.

This design is commonly available with discrete VHF and UHF sections combined on the same beam, in effect, two antennas in one . This is a good choice for VHF III and UHF IV & V bands.

One antenna that is particularly suitable for RV use is a model from Tandy Electronics, Cat No 15-9706 and priced at $A95-95 (June 2001).   It is quite well made (and in Australia) and covers channels VHF 6-11 and UHF 28-69.   Unlike many, all tubing is seamless aluminium.   This is the largest antenna I would like to have to set up on a daily basis.

Photo on the right shows this model.
It can be easily assembled such that the VHF and UHF can be independently set for either horizontal or vertical polarisation.
The one pictured has both set for horizontal reception.

The ability to align for horizontal and vertically polarised signal is very important if you are to have any hope of extended coverage around the country. 

It is shown fitted with a GME Kingray MHW34G masthead amplifier which can be had for $A80 to $A115 depending on where you buy it (Oct 2003). This is a special version of the MHW34 that is modified by GME to operate from 12V DC for marine and RV use.

I am presently testing this antenna / amplifier combination and while first impressions look good, I will comment on its performance in the near future.

I consider a VHF version for VHF I & II bands to be too large for convenient use in caravan applications, not only cumbersome but too easily damaged.

A UHF only version for UHF IV & V would make an excellent choice for these bands.   Once again, Tandy Electronics have a well made (also Aust made) model,  Cat No 15-9811 for $69-95 (June 2001).   This one is quite compact, convenient and quick to set up.


The Log periodic

Another multi element antenna visually resembling the Yagi but giving a greater bandwidth is the Log-periodic antenna.   It differs from the Yagi in that all elements are driven and yet also act as directors and reflectors depending on the frequency of operation.

If correctly designed (and this is the problem), the log periodic with its' gain and greater bandwidth is the best choice for VHF III and UHF IV & V bands and may be a little more compact than the combined Yagi above.


Log periodic Yagi

This is a log periodic design that incorporates parasitic elements.

It offers the gain of a Yagi and bandwidth of a log periodic, but its' physical dimensions would generally preclude its' use with caravans etc.


Phased arrays

Put simply and in a TV antenna context, it is a series of dipole and reflector combinations, stacked vertically, usually two to four high to form one antenna.   It makes an excellent choice if designed for UHF IV & V band reception but a VHF version would be too unwieldy for caravan use.

You will also encounter antennas comprising Yagi / phased array combinations, which if correctly designed also make a good choice for VHF III and UHF IV & V bands .


 Summary

Correctly designed these multi element beam antennas offer much higher gain and offer far better performance than so called omnidirectional and half wave dipole antennas.   This is governed by the law of physics, to use the light bulb analogy again, a bare 100 Watt light bulb will never, no matter how cleverly packaged and promoted, out perform the same reflector focused 100 Watt bulb.

I would avoid vendors who claim theirs is compact all band antenna that will perform as good (or better) than directional antennas dedicated to specific bands.    I have witnessed some outrageous claims for tiny 'all band', often 'omnidirectional' antennas.    They may look appealing due to their small size and ease of installation but unless you are close to the TV transmitter, my advice is to look elsewhere.


 Polarisation

TV transmissions can be either horizontally or vertically polarised.

Without going into detail, horizontally polarised transmissions are received with the antenna elements oriented such that they are horizontal with reference to the ground, similarly vertically polarised signals require the antenna elements to be in the vertical position.
The above photo shows that antenna set up with both the VHF section and the UHF section set up to receive horizontally polarised transmissions.   However, it can be readily adjusted so that one section is set up to receive horizontally polarised transmissions and the other section set up for vertically polarised transmissions which can be very useful.

Unfortunately, TV signals in Australia are a mix of horizontal and vertically polarised transmissions.  The selection of an antenna needs this to be taken in to account.

As I said earlier, the ability to align for horizontal and vertically polarised signal is very important if you are to have any hope of extended coverage around the country. 

(The theoretical separation between horizontal and vertically polarised signals is infinite, but due to the effects of propagation over the land mass, in practice it varies in the vicinity of 10-15 dB.)

Aligning the antenna at 45deg  to the horizon is useful if you are in situation of having to receive both, but you will loose 3dB or half of the available signal in doing so.


 Amplifiers

Because we have had to compromise with our antenna design to keep it to a manageable size and also because we are invariably traveling to poorly serviced regions, a good masthead amplifier can be essential for analogue TV reception.

I have heard it said on a number of occasions, often by people that should know better, that an amplifier is not necessary in a caravan type of situation because an amplifier is only used to compensate for the loss of the coax cable which due to the short run is not significant in this case.
Whilst it is true that the amplifier does make up for the loss of the coax which isn't much in this case, it may be needed for another very important reason.
A major limiting factor in receiving a weak signal is the inherent noise generated in the front end of the TV receiver.   This inherent receiver noise masks the low level signals. 
A good masthead amplifier often has a lower noise figure than the TV receiver, some times considerably lower.  
Note1The dominant factor in determining the system noise figure is the first amplifying stage and the lower the overall system noise figure, the lower the signal level that can be retrieved. 

For analogue TV, a good low noise masthead amplifier can often mean the difference between a good picture and nothing.

However, (in the context of receiver noise not feeder cable loss) the improvement achieved by an amplifier is a factor of the quality of the amplifier and of the receiver (TV, STB or VCR).
To achieve an improvement the amplifier must have a lower noise figure than the receiver, usually much lower to be worthwhile.
Typically consumer quality receivers have noise figures in the order of 8 to 15dB (although the front end of digital receivers seem to be generally better than this) and wide-band amplifiers of the quality usually available for this application in the order of 4 to 8dB.
If you have a receiver with a noise figure of 15dB and use an amplifier with a noise figure of 4dB, you will achieve a major improvement in receiving performance but if you have 8dB receiver and an 8dB amplifier there will be no improvement whatsoever.
Consumer quality receivers rarely specify this data so the question of whether to try an amplifier is, in the absence of dedicated test equipment, hit and miss.

In any case, a masthead amplifier should always be used with a good antenna, not as a substitute for one, always start with the best antenna possible.

However, don't use a masthead amplifier for digital TV reception unless absolutely necessary and if you must, only use one with variable gain and turn it down to minimum.


The amplifier will consist of two components, the masthead unit, and the power injector.

The mast head unit is the actual amplifier and should be mounted as close as possible to the dipole element of the antenna.

The power injector is mounted inside your RV and has three connections.

-   12 VDC input from caravan electrics.
-  

Coax cable to the masthead unit (sends DC voltage to the amplifier and also receives TV signal from amplifier), preferably in one continuous unbroken run, i.e. no intermediate plugs, sockets or connectors.

-  

Coax cable to TV or VCR, (preferably either one or the other, not both using a splitter).

 

If possible check amplifier specifications, I would be looking for the following,
 

  Analogue Digital TV  ( if you must )
-    Frequency response  45 – 820 MHz  175 - 820 MHz for Aust digital TV
-    Gain  15dB or higher  6dB for digital TV
-    Noise figure  5dB or lower 4dB or lower for digital TV
-    DC Operation 12VDC 12VDC


 Coaxial cable

Use only a low loss RG-59  75 ohm coax with air-spaced dielectric or foam-air dielectric, and keep the length as short as practical.   This is particularly important at UHF frequencies.

Also the fewer joins (i.e. coax plugs and sockets), the better, and avoid getting water in the coax at all costs.

 Splitters

Avoid using them, they all introduce a loss.   More, sometimes much more than 3dB or half of the signal that you have tying to maximise can be lost in a single two outlet splitter.    If you must connect two or more appliances simultaneously, use an additional multiple outlet amplifier which compensate for that loss. 


 Conclusion

It is not possible to purchase one antenna of a convenient size for use with a caravan that covers all five bands effectively. (I am talking here about receiving a weak distant signal, not a strong local one.)   No matter what you choose it will be a compromise of size verses performance. 
  
Personally I use a Winegard RV3090 antenna shown on the right.   I believe it is the best overall compromise, particularly where convenience and ease of use is a priority.

Although it is a simple half wave dipole, it is quite effective in a medium signal strength area.  When not in use it lays flat on the roof.   On the ceiling inside the caravan is a small crank handle to raise and lower the external antenna.   A concentric ring around the crank handle rotates the antenna to the best signal position.   This is all done from within the caravan while watching the TV.   The antenna incorporates a built in amplifier.

In the context of what is practical for an RV, I would give it 5/5 for convenience and 4/5 for performance.

I also carry a VHF/UHF Yagi in conjunction with a high gain low noise amplifier I built, for the occasions when the Winegard is not suitable.   Rating for that, 5/5 for performance and 1/5 for convenience.   In practice, I would probably only use this 10% of the time.

To conclude, in my experience, the table below would represent a fair and unbiased comparison between the antenna types.   As with everything, there are good and not-so-good products and I am trying to portray an average using the VHF/UHF Yagi shown above above as the reference.
The figures below are without amplifier / with amplifier, with 5 being a very good picture an 0 being an unusable one on average across the TV spectrum and assuming amplifier is a good stable low noise high gain type.     I am assuming a receiver of average quality, see my remarks above.

Strong
Signal

Medium
Signal

Weak
Signal

Omni directional 5/5 1/2 0/0
1/2 Wave Dipole 5/5 2/3 1/2
Yagi 5/5 4/5 3/5

Update Dec 03

I do recommend the GME Kingray MHW34GLV which I mentioned earlier for analogue TV reception. 


 TV Transmitter Locations

June 08

It is obviously very useful to know where the TV transmitters are, if any, in the area you are visiting.

However, due for frequent changes on government web sites, my previous links to TV location data needed constant updating.   Therefore I am deleting external links to this information.

In place of the previous information and links I have now substituted some very useful files provided by Ian Gliddon.
Ian has converted a published list of Aust TV transmitter locations such that they can be displayed on OziExplorer as waypoints.   ( See my GPS page.)

This give you the ability to determine an exact bearing and distance from your location to a particular TV transmitter.
With that information and the power and pattern info also provided you can determine the likelihood of TV reception at your location.

This info is in the form of five files including the original pdf file that can be read as a stand alone file.  An instruction file is also included.    The five files total 1.36Mb.

Due to web space constraints (this is a non commercial web site), you can email me to receive the pdf file and  if you are a registered OziExplorer user I will also send you the other files.


Digital TV and  Digital Receivers (or Set Top Boxes)

Sept 05

Just a brief mention on the subject of digital TV.

Digital TV, why bother I have been asked.

Well, a far better picture, no noise, no ghosting and if that isn't enough, soon you won't have any choice, analogue TV will be no more.

How do we receive digital TV, well for most of us we will need to purchase a Set Top Box.  This is a digital TV tuner.   You simply connect your antenna to the STB and then a cable from the STB to the TV.    Prices start from under $100.

Besides better picture quality the STB will provide for viewing of an on-screen programming guide, sub titles, teletext, on-screen channel signal strength, choice of screen formats (normal 4:3 and wide screen 16:9) and a host of other features.
Most will have multiple video and audio outputs for stereo and surround sound and a serial port to enable upgrading of the system software.
They are very easy to set up and tuning is automatic (simpler and quicker than tuning our TV or VCR/DVD combo).

Best to choose a model that operates from 12VDC, most are AC only.

My choice after much research was the Strong SRT5005 which operates from 12-24VDC and 100-240VAC. and RRP is $179.
(If this article causes you to purchase a Strong STB, do not purchase the SRT5000 or the SRT5006, they are both AC only and do not have a DC input jack.) 

This model has on the front panel, on/off switch, channel up/down, status light, LED signal strength indication, four digit display for channel selected (and current time when in standby mode) as well as scrolling messages telling you it's scanning or booting up.

At the back there are connectors for antenna in and loop out, composite out, s-video out, component out, stereo audio out, digital audio out, RS232 serial port for software upgrades and a DC input jack.

It is supplied with a switchmode 100-240VAC > 12VDC plug pack.

The remote control is logical, my only criticism is the unnecessarily fast repetition rate when a key is held down, (you have got to be quick to get your finger off the button). 

The instruction manual is concise but adequate, missing out only on explaining a few minor points.

Setup was easy and the on-screen menu is simple and for the most part, logical.
Channel scanning is performed quickly and easily.

Feb 07 update

I recently purchased a very compact STB marketed under the AWA brand from Big W.

This unit is very well built and rugged, the case being extruded aluminium and the full function remote has a seamless key pad rather than individual buttons.
The STB measures only 105 x 63 x 22mm and the remote is 117 x 55 x 13mm.

The front panel has channel and volume control, menu select button, standby LED, signal lock LED, remote control IR receiver and a combined data port (for software upgrades) and an IR remote extender (enabling you to mount it out of sight).
The rear panel has an antenna socket, two AV ports and a 12VDC input jack.

It is supplied with a 100-240VAC switchmode power supply, a 12VDC cigarette lighter plug, an IR extender and a magnetic base antenna. 

Only composite video output is available but I can hardly pick the difference between composite and S-Video anyway.

Performance is on par with the Strong described above providing you only need a composite output and at $69 is very good value, all in all, an impressive bit of gear.

Sept 08 update

Time marches on and the above Strong has now been replaced by a high definition STB.   This time it is a reasonably compact one from Dick Smith marketed as a DGTEC for $99.

Unfortunately, 100-240VAC only (I have modified mine to accept 12VDC as well) but that aside, since all aspects of performance is good, I have no problem recommending this STB.

(If you get one of these, you must press the "Format" button on the remote the first time you use it so that it is compatible with your TV.  The manual omits to tell you this.)

 
Sept 11 update

By comparison, we are spoilt for choice now.  My previous STBs have been replaced with a Strong SRT5431.
Like all things electronic, price, size and power consumption is down and features are up.
The latest crop of STBs have a USB connection allowing programmes to be recorded directly to a hard disk or USB memory stick.

This latest Strong ($49-95 at Big-W) is a gem.  My only criticism is a ridiculously directional remote control requiring extremely precise aiming at the STB (very annoying). 
Since the remote can't be easily pulled apart to replace the infrared LED with a wider beam-width one without risk of damage, I fixed this simply by scraping the remote's LED lens with a knife blade to allow scattering of the infrared beam - works a treat now.

I use this STB in conjunction with a pocket size 500Gb USB hard disk (about $55) and the saved programmes can be viewed an a TV (with the STB) or directly on my lap top computer.

Although AC operation only (100-250VAC) it only consumes 8W.


In response to several emails asking if it is possible and practical to view TV on their laptop, I have added this section.

The short answer is yes and I do.    There three ways of doing so.

One method is to use a USB tuner.    I have two of these, the one that was included with my laptop top, which is absolutely useless and the other which gives excellent results. 

The latter is a Dvico Fusion HDTV USB tuner with a remote.
The hardware is very good quality and receiver sensitivity is excellent.   The software is also excellent and upgradeable from their website.     This tuner is powered from the USB connection. 
It gives you the ability to watch both standard and high definition TV. (Even both of the TV channels HD and SD transmissions simultaneously with a picture-in-picture feature.)
Using it is simply a matter of connecting it to your USB port and to an antenna and clicking on an on screen icon.
The software also gives you the ability to save programmes on your hard drive.

While this setup is easy to use and gives excellent results, the down side is price, about the same as a 15" LCD TV. (Jan 08)
However, if you want to travel light using your laptop as a TV and recorder, this is an excellent way to go.

The second method is to use the output your present set top box or VCR.

To do this you will need a video capture device.  This gadget will accept a video signal from the STB or VCR, either composite or S-video and output to your USB port.   The audio will usually be fed directly into the mic jack on the computer.     Once again, you can save programmes to your hard drive.

The down side is inferior picture quality compared to the previous method above and secondly, a bit fiddlier to set up and use and messy cables.

A K-World USB video capture device is an example of one for around $75.   Also useful if you want to put any of your old video tapes on to a DVD.

The third and best way is to purchase a laptop with an inbuilt HD TV receiver.

Irrespective of the video source, you need to be aware that whether the picture on your laptop screen is as good as a good LCD or plasma TV will depend and the quality of the laptop LCD.   

While the computer screen resolution may be adequate, often laptop screens have a longer refresh rate than a good LCD TV which may cause the action part of scenes to be a little jerky.
Also, contrast ratio may not be as good causing a slightly washed out look in a side by side comparison.
Not a significant problem in practice especially considering the time you will most likely be using it.

However, if you are happy watching a video DVD on the laptop then it's good enough.    Most newer laptops will give good results and getting better all of the time.

Be aware that there are minimum computer hardware requirements such as processor (CPU) type and speed, amount of memory (RAM) and hard disk space available particularly is you want to record programmes.
Once again, most if not all newer laptops since 2007 should be more than adequate.


Update Sept 11

Our present TV viewing on the road is now done on our ASUS laptop computer screen using the above Divco HD tuner.   We also take along the 500 Gb HD described earlier, which has programming from home done on the Strong STB.


 Satellite TV reception

For those of you who must have TV reception wherever you go, you could consider satellite TV reception.   Mobile systems are available for free-to-air TV with prices ranging from under $400 to over $3000.   I plan to go into this in detail a little later when time permits.


Mar 11 Update

Judging by quite a few emails I have received, many seem to be having problems with their mobile TV satellite systems prompting me to soon cover this subject in some detail in the near future - time permitting.

This matter has been brought to my attention on quite a few occasions recently by purchasers of systems that do not work as well as advertised and some so difficult to set up that they are never used.

These systems comprise of a parabolic dish with some sort of mounting, and LNB, a satellite finder and a receiver.   It seems the quality and capability of components vary depending the vendor as much as the price.

End


  
Note1 

    The combined (cascading) noise figure (MH amp + TV) is derived from the following formula . . .

      
N total = N1 + ((N2 - 1)/G1)

          where

N1 = masthead amp noise figure
N2 = TV receiver noise figure
G1 = masthead amp gain

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