How do I know if i have RF in the shack
I keep learning and asking question but now and again I come against a stumbling block which prevents me progressing so i have to turn to my fellow HAMs.
I understand RF is bad, How do i know if i have it and what will be the symptoms and how can i prevent it?
[[[Can i open my eyes now]]]
To see if you have any RF on any of your cables in your shack, get the MFJ-854 current probe.
Here are just two simples steps, out of many, to clean up RFI issues.
1. Use a balanced antenna with good coax.
You can get a field strength meter.
That is not a correct characterization, RF is what you desire --
I understand RF is bad ...
BUT it must be managed or contained (like water) or it will go everywhere!
Coaxial cable is an UNBALANCED transmission cable -- BUT the 1/2-wave dipole (Hertz antenna) is a BALANCED radiator (load).
The 1/4-wave vertical (Marconi antenna) is unbalanced, BUT has a lower impedance (Z) that must be properly matched.
Electronics for Radio Amateurs (EET-241)
Module C - Antennas, Propagation, and Transmission Lines#
Transmission Lines - Introduction
Practical Antenna Handbook by Joe Carr (695 pages)
Last edited by W9GB; 04-25-2012 at 12:58 PM.
We keep moving forward, opening new doors, and doing new things, because we're curious and curiosity keeps leading us down new paths. -- Walt Disney
You don't need a probe. Just waving a flourescent tube along the outside of your co-ax when keying up will tell you if there is any.
Originally Posted by KU3X
As for the advice:
1) Yes, a balanced antenna is good however its very hard to do even on a dipole as especially in the UK there will be nearby objects that affect the antenna meaning the current flowing in both legs is not equal.
2) Ferrites if you have a poor RF ground. If you're running a vertical antenna, you need radials. If you look at http://www.karinya.net/g3txq/chokes/ it gives you some turn ratios for different toroids to make your own RF chokes. For most people, 8 turns of RG58 on a FT240-31 toroid will be sufficient. To make one it is as simple as wrapping 8 turns of RG58 round the toroid with a connector on each end. Make two and have one at the antenna and the other where your co-ax enters the shack.
If you are transmitting RF with an external antenna then you will have RF in the shack, unless your shack is built as a Faraday shield. So the question is how much RF is showing up in your shack and what equipment do you have that is sensitive to it, and if so how best to deal with it.
Reality is that which, when you stop believing in it, doesn’t go away.
I think that part of the problem with your question is that it uses ham jargon which leads to imprecise interpretation.
This phrase "RF in the shack" can mean many things to different people.
RF energy radiates from your antenna through space. Some of this space includes your shack. Whether this is a good or bad thing depends on how that RF affects equipment and objects in its field.
Some of the bad things are:
RF feedback. - An example of this is rectification of the RF energy in the audio input to your transceiver. This can cause garbled speech or distortion in voice modes. It can cause impairments in audio based digital transmissions. It can cause the transmitter to continue transmitting with no additional audio input when in VOX mode.
Transmission line radiation. -You want your antenna to radiate and not your transmission line. If a significant portion of your energy is radiated by your transmission line, your antenna may not work as intended. Some of the energy never gets to it and the pattern may be corrupted due to this radiation.
RF Safety - RF is non-ionizing radiation, but there are limits to what is safe for long term human exposure. If you have RF radiation above these limits due to running high power or high field intensity with the antenna too close to your station you may be causing heath related risks. Run a calculation to see if everything is OK.
RF Susceptibility - If the RF field is too strong in your station, house or neighborhood it may cause malfunction in consumer electronic devices. Much of this is due to inferior design of those devices. While you can apply some filtering to these devices if they are yours, it is sometimes easier to move the antenna farther away from the affected device or lower your transmit power or some combination. This is especially true if the device belongs to an neighbor you don't wish to irritate.
RF Interference - Because an antenna is a reciprocal device it will receive signals emanating from your house as well. Many consumer products have been manufactured with inferior emissions properties. Some of these may meet regulatory requirements and some may not. But the fact of the matter is that they often create signals or noise which interferences with your reception of radio signals. Moving your antenna farther from these noisy devices and being able to orient it so that it minimizes the effects of these interferers is becoming more important with the proliferation of these RF polluters. If you want to receive weak signals, you need to minimize the local noise and interference.
So if you could minimize all of these negative things that people often call "RF in the shack" you would have a more effective station. But like most things in life there are tradeoffs to be considered. Having some station is better than becoming paralyzed by over-thinking every aspect and having none. There will always be some RF in the shack. If you don't have any of the problems mentioned above enjoy your radio.
You will soon know when you have RF in the shack, a hot microphone, power supply could switch off, computer squeals like a pig, software goes beserk, things appear when they shouldn't.If you don't have RF at some time you are one of those blessed by the one above !