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Wednesday, 30 August 2017

Causes of poor radio reception

(This was posted to the SA-Scan group and is reproduced with permission)

As the 2 way radio scene is rapidly changing from analog to one of the many digital transmission
formats available and the SA GRN network will be switching to a full digital system in the near future there is going
to be new challenges in receiving good quality digital transmissions and opinions will always differ, there is sometimes no one singular fix to receive a good quality digital transmission if you are receiving corrupt transmissions in some cases.
This is a guide to possible causes to poor 2 way radio digital transmission reception and is open to many different
opinions, agreements or disagreements and may help fix reception issues you might get with digital transmissions.

The scanner or radio receiver.
The scanner is often the biggest weakness as the majority are not true narrow band receivers by 2 way radio standards.
In Australia the standard bandwidth for VHF & UHF band 2 way radio is 12.5 kHz channel bandwidth with 25 kHz still
allowed in remote locations, most digital transmissions are 10.8 kHz bandwidth with a little spare on the side.

Most scanners have a 30 kHz channel bandwidth which allows for adjacent channel bleed through, this is the most common cause for interference and affecting how well digital transmissions are decoded, now in analog we would hear that interference but as long as we can hear what is being said we tend to ignore the interfering noise, digital on the other hand if that interference is strong or noisy enough it will cause the received digital transmission
to start chopping in and out (breaking up) or not to decode it at all, there is no real fix to overcome this and the problem
is more prevalent when monitoring multi site trunking networks where there is a lot of adjacent channel transmissions,
This is also more common in large metro area's where there is a large amount of 2 way radio traffic and less problematic
in rural but can't be ruled out.

Electrical interference.
Household.
This can be caused by many different sources, from poor quality LED lighting to cheap E bay electronic goods
but even quality manufacturers can even have the odd problem product, rooftop solar systems where the manufacturer has used poor quality electronic components that give up in the grid interactive inverter and allow noise in the electrical system.
Earthing faults within the household though rare can happen and earth loop back noise will cause problems.
Most often these can be identified by a buzzing noise when listening to analog transmissions.
Sometimes the interference could be coming from a neighbours house or 2/3 away.

Motor vehicle.
Even in today's modern vehicle electrical interference still happens from alternator, ignition and electronic PWM
ventilation fan speed operation, some vehicles radiator fans are even PWM variable speed control,
poorly manufactured electronic circuits on after market HID and LED lighting is the biggest culprit.
Some alternator voltage regulators can cause noise when under heavy electrical loading.
Also earthing faults within the vehicle is another common fault.
It could also be coming from a vehicle near you while travelling.

The receiving antenna.
In the household environment the most popular type is the discone due to it's wide bandwidth of frequencies it can cover.
It has it's pro's and con's but depending on your location it can either reduce or contribute to RF interference,
this is a tricky one where there will be difference of opinions.
A dedicated frequency band antenna again has both pro's and con's like the discone but limited to the frequency band it's designed for and will perform very poorly outside of that band.
The old height is might saying, it is generally true but again if you are in a heavy RF area it can actually increase interference as the antenna becomes more exposed to RF signals from more sources.
The gain antenna, again more difference of opinions here, it can help but it can be a cause to more interference being received.
On the vehicle either an average frequency trap wide frequency band scanner antenna or a dedicated frequency band antenna not much give here but in heavy RF environments the scanner will suffer no matter what.

Handheld scanners often are provided with either a basic antenna tuned for a centre frequency that the scanner will cover from lowest to highest frequency and not work to well outside of the centre frequency, some might come with a
compromised band trap type, if you are using the scanner at an event where the frequencies you are monitoring are
centralised a frequency dependant band type antenna might be a better option.

Coax cable on a base set up.
Using the wrong cable can possibly lead to interference, also using RG58 over a long run will also cause heavy attenuation of the incoming signal thus creating a cause for poor signal strength and quality at the receiver, also use quality connectors and limit as much as possible the amount of connectors or adaptors used, this always lead to data quality issues as each connection point adds attenuation and in rare cases can be a cause for an interference point to happen.

Do you need to have a look at your existing set up, yes you may have to, what might have been OK for analog might
be an issue to receive good quality digital transmissions, this is to help there is some cases due to your location
you may have interference that you will not be able to eliminate.

There are members on this group who have vast experience in the RF field and no doubt would be willing to help,
never be afraid to ask we all started out green and inexperienced at one stage but there will be difference of opinions on some of the above.

There is most likely other points I haven't covered here but it is a starting point.
Happy Scanning.

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