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HAM radio turns off TV cant find out why..

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ZakWolf

Thu Jul 21 2011, 10:20PM

Registered Member #3114 Joined: Sat Aug 14 2010, 08:33AM
Location:
Posts: 608

When my grandpa uses his HAM radio, it makes his TV either turn off or makes it change channel etc... His radio is about 12 ft away from the TV and the gci receiver. He replaced his HD DVD player twice and his gci receiver twice; he has used RF filters and has also changed the length of the cables that his TV uses. He has a 20m yagi beam mounted antenna on the roof, he has used two different types and brand of HAM radio.

His main frequency that he uses is the 20 meter band, and the 15 meter band. (20m band is 14.00KHZ)

The 20 meter seems to affect the TV quicker than the 15 meter band.

Any idea what could cause this?

Proud Mary

Thu Jul 21 2011, 11:23PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

At such close range, so-called 'direct detection' can become an issue.

Circuit elements not intended to demodulate RF will do so if there is enough of it from a nearby transmitter. Signals can appear in unwanted places because potentially any part of the TV circuit can act as a receiving antenna, and almost any IC will rectify the unwanted signal, sometimes creating switching voltages where they are not desired.

A cure may not be easy, because consumer TVs are not designed to operate in strong RF fields, nor are consumer transmitters necessarily designed and matched to ensure that harmonics - multiples of the intended transmitter frequency - are only radiated at ultra-low levels.

More and better shielding, screening, mains filtration, and trap filters in the TV antenna input are really the only way to go if the TV must be operated in close proximity to the transmitter, and its transmission line and antenna. Proper matching of transmitter and TV coaxial transmission lines should also be investigated. For example, the transmitter coaxial line may be radiating when it ought not to be. There may be unwanted coupling between the two coaxial lines, and so on. Improving the decoupling of the supplies to the affected sub-circuits in the TV may also bring about improvement.

Summary verdict: the unwanted switching action described strongly suggests direct demodulation of the transmitter signal by switching circuits in the TV.

ZakWolf

Fri Jul 22 2011, 08:27AM

Registered Member #3114 Joined: Sat Aug 14 2010, 08:33AM
Location:
Posts: 608

thanks Mary :)

Steve Maurer

Thu Jul 28 2011, 02:22AM

Registered Member #133 Joined: Fri Feb 10 2006, 10:27PM
Location: Pensacola, Florida
Posts: 47

FYI - The 20-meter band is 14 MHz.

Fixing the issue at the television itself may not be the best way to approach this problem.

As there are a number of things that could potentially cause the issues described, answers to the following questions may help point us in the right direction of the source of the problem:

1) What type of RF transmission line (antenna feedline) is being used between the radio transmitter and the antenna? (i.e. â€“ coaxial cable, ladder line?)
2) Is an impedance match (also called an antenna tuner) being used between the radio transmitter and the transmission line? Is a balun (impedance matching device) being used between the transmission line and the antenna?
3) What is being used for the radio grounding conductor (wire, copper ground strap, wire braid / and what size is it)? How long is the radio ground conductor? What is the other end of the radio ground conductor fastened to?
4) How far is the radio transmitter antenna from the television? In what direction is the yagi antenna facing? Is it facing the television?
5) What is the expected transmission power of the amateur radio transmitter?
6) How is the television receiving its TV signals? (i.e. â€“ cable TV, rabbit ear antenna, external roof antenna?) Where are the cable TV or antenna cables routed and are they in close proximity to the radio transmitter feedline?

If the antenna is a fair distance from the television and is not facing the television, then RF energy is likely radiating from the transmission line, ground lead, and/or radio chassis due to problems relating to an RF impedance mismatch or a grounding issue.

I would check for impedance mismatches between the radio and the RF transmission line and also between the transmission line and the antenna. The RF grounding for the radio should also be examined. (A short, wide, copper ground strap directly routed to a properly installed earth ground rod would be best.)

In a transmitter system that is configured properly, there should be little RF energy radiating from the transmission line. In an ideal system, all of the RF energy should radiate from the antenna.

One way to test for high levels of RF emanating from the cables would be to use a disconnected neon bulb that is attached to the end of an insulating stick. (The insulating stick is for safety, especially if 600-Ohm bare wire ladder line is being used with a high power transmitter.) Place the bulb in close proximity to the transmission line and check it at several locations across the length of the line. If the neon bulbs glows, then you have a fair amount of RF energy radiating from the transmission line. You may also check the radio and the ground lead with the neon bulb.

Note that even the radio ground conductor itself may radiate if the system (radio, ground, antenna, and feedline) is not configured properly.

If you know any fellow radio amateurs that have a decent RF field strength meter, you may also use it to locate the offending RF radiator source in the system.

- Steve

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