Taming your stray RF that causes intermittent SWR reading by using RF Choke


Antenna system works depends on a lot of variables. Radio signals are affected by antenna efficiency, feed line loss, nearby objects, terrain, weather and a lot more factors that are beyond our control. It is difficult to control all factors but we can limit ourselves to optimizing those things that we can control. Like limiting stray RF from travelling to our feed line and cause an intermittent SWR reading.

SWR or Standing Wave Ratio is a measurement of antenna efficiency. When you are transmitting you are sending Radio Frequency along your antenna feed line. The antenna then converts this RF energy to Electro Magnetic energy which is radiated into space. If the antenna and feed line are not working efficiently some of this energy is reflected back to your transmitter along the feed line, thus it becomes wasted energy.

SWR is the ratio between transmitted or Forward energy to the Reflected energy. This ratio can be computed as:

SWR = Forward + Reflected/ Forward – Reflected

The most common piece of test equipment used to tune and test antenna system is an SWR meter. This tool can give you a lot of information about antenna, and can help you to tune your antenna’s resonant frequency, adjust antenna impedance.

So normally when we tune our antenna without using those expensive antenna analyzer (they are handy when doing frequency sweep analysis) we just use our trust worthy SWR meter coupled with our hand held radio and insert the SWR between the radio and our antenna system. In tuning the antenna we select the center operating frequency that we desire for our antenna to have a lowest SWR reading (resonance). We normally set the radio to the lowest transmit power to tune the antenna and to limit the interference we might cause while tuning. Once the acceptable reading is achieved we can expect that the tuned antenna will perform the same or with very minimal deviation on the SWR reading even if we increase the power applied to feed line and the antenna.

Sometimes this is not the case and we can expect a linear increase in SWR measurement with respect to increase of power applied to the antenna system. The way choke work is simple, a small amount of RF radiates from the shield wire in the coax. RF emitting from the antenna surrounds the antenna with a very large pattern filled with RF. Some of it gets on to the coax and can travel down the entire length of the coax, additionally a slight mismatch will cause some RF to be reflected comes back down the surface of the coax.

The RF choke creates an electromagnetic field on the chokes surface, and within the donut hole. This field attracts the stray RF and chokes it off before it travels down the coax, and it is dissipated within the electromagnetic field. Hence the term “Choke”. In some ways, it almost acts like a ground radial and reflects slightly the RF radiated from the antenna, to a more upwards angle sending more of the signal towards the horizon. But unlike a ground radial, the choke does not tune the antenna to any specific frequency. So it is good for all frequencies from 160 through 6 meters.

Here’s a video link demonstrating effects of RF choke on SWR reading, using the home brew choke I made. Pardon the audio it’s not meant for broadcast

Feel free to comment, constructive criticisms are very much welcome:
(Please read the provision for the use of Emergency Channel assignment in the 2m Philippine amateur band)

The center frequency of Philippine Amateur Band (Amateur Radio Service) for 2m happens to be 145Mhz the frequency assignments is from 144 – 146Mhz primary.

The center frequency is currently assigned to
Emergency Channel the channel assignments mentions the following provisions:

3.1 Channel 145.000 Mhz +/ 25KHz shall be used for emergency communication and general calling for radio telephony.

3.2 To facilitate the reception of distress calls, all transmission on 145.000 Mhz shall be kept to a minimum and shall not exceed one minute.

3.3 Before transmitting on the frequency 145.000 Mhz, a station should listen on this frequency for a reasonable period to make sure that no distress traffic is being sent. This provision does not apply to a station in distress.

The above video are for educational purposes only and there was no intention of breaking the above provisions on my part. However since these videos are made using the center frequency as reference it is good to note that the same frequency assignments can be used for general calling for radio telephony. (General; in my interpretations means the same frequency can be used provided that there is no distress traffic going on the channel (subject to other interpretation), and the transmissions shall not exceed one minute (must be 1 minute continuous transmission without gap), and care has been taken to listen carefully on the frequency to be sure that there is no distress traffic being sent.

3.4 Distress communications

3.4.1 The distress call sent by radio telephony consists of:

-The distress signal MAYDAY spoken three (3) times
-The words THIS IS (or DE spoken as DELTA ECHO in case of language difficulties).

3.4.2 The radio telephone distress message consists of:

• The distress signal MAYDAY.
• The name, or other identification of the mobile station in distress.
• Particulars of its position.
• The nature of the distress and the kind of assistance desired.
• Any other information, which might facilitate the rescue.

3.5 Any station in the amateur mobile service which has knowledge of a distress traffic and which cannot itself assist the station in distress is forbidden to transmit on the frequency in which the distress traffic is taking place, until such station receives the message indicating that normal working may be resumed.