Hello everybody and welcome back to Everything Hamradio! Today we are going to continue our series on the Technician Class License Question Pool! Today we will be talking about different Coaxes and SWR Information.
As always, the correct answers will be in bold. I also recommend that you only read the correct answers when studying for your test. If you do this, when you take your test and you see a question, there will be a better chance that the correct answer will jump out at you easier. Some say that this approach is kind of like cheating, but the way that I see it, you will always be learning something in this hobby and you don’t have to know everything there is to know about the hobby to get your license.
If you would like to purchase a copy of the Technician Class study book, written by Gordon West, WB6NOA with Eric P. Nichols, KL7AJ, that this series is loosely based around, below is a link to it.
Let’s move on to the good stuff, shall we?
T7C12 Which of the following is a common use of coaxial cable?
- Carrying dc power from a vehicle battery to a mobile radio
- Carrying RF signals between a radio and antenna
- Securing masts, tubing, and other cylindrical objects on towers
- Connecting data signals from a TNC to a computer
Coax is used to carry RF signals between a radio and antenna. While there are other answers that appear basically the same, the key word in this answer is RF. Answer A is carrying power, and D is data. Answer C is just plain crazy, cause coax can be expensive and its not always very bendable.
- It is easy to use and requires few special installation considerations
- It has less loss than any other type of feed line
- It can handle more power than any other type of feed line
- It is less expensive than any other types of feed line
With coax it is easy to use and doesn’t require very many consideration when installing it. Think of it like a garden hose, try not to kink it, scrunch it, or squash it so that the flow of energy is not broken or limited.
T9B02 What is the impedance of the most commonly used coaxial cable in typical amateur radio installations?
- 8 ohms
- 50 ohms
- 600 ohms
- 12 ohms
In the amateur radio community, the commonly used coaxial cable is rated at 50 ohms. If you do something to damage the coax, like slamming it in a car door, it could distort the shape and cause a mismatch in impedance and therefore damage your radio or at the least, limit your power output.
T9B05 What generally happens as the frequency of a signal passing through coaxial cable is increased?
- The apparent SWR increases
- The reflected power increases
- The characteristic impedance increases
- The loss increases
As frequency increases so does the loss of signal. The higher frequency you go, the more loss you will have, so you will need larger coax to cope with the loss. While there is some coax that you can get that are small and have less loss, they don’t always perform the way that they should. I generally use RG-8 or better in my installations that are not in my vehicle.
- They are preferred for microwave operation
- They are water tight
- They are commonly used at HF frequencies
- They are a bayonet type connector
PL-259 connectors are generally used for HF frequencies but they are also used on mobile and base 2m/70cm radios too. When you get into the higher frequencies is when it changes.
- A UHF (PL-259/SO-239) connector
- A Type N connector
- An RS-213 connector
- A DB-25 connector
Even though most mobile 440MHz radios come with PL-259/SO-239 connectors on them, a general rule of thumb is anything above 400 MHz should use a Type N connector.
- It has more loss per foot
- It cannot be used for VHF or UHF antennas
- It requires special techniques to prevent water absorption
- It cannot be used at below freezing temperatures
Air core coax is good, but it is more trouble than it is worth in the amateur community. Mainly because your have to use special techniques to prevent water absorption
T7C09 Which of the following is the most common cause for failure of coaxial cables?
- Moisture contamination
- Gamma rays
- The velocity factor exceeds 1.0
If water gets into your coax, it makes for a lousy signal. You should always seal any exposed coax connectors up at the antenna feed point. The PL-259 connectors are not water-tight!
T9B08 Why should coax connectors exposed to the weather be sealed against water intrusion?
- To prevent an increase in feed line loss
- To prevent interference to telephones
- To keep the jacket from becoming loose
- All of these choices are correct
When water gets into your coax, it could make your signal lousy, because of increased fee line loss.
T7C10 Why should the outer jacket of coaxial cable be resistant to ultraviolet light?
- Ultraviolet resistant jackets prevent harmonic radiation
- Ultraviolet light can increase losses in the cable’s jacket
- Ultraviolet and RF signals can mix together, causing interference
- Ultraviolet light can damage the jacket and allow water to enter the cable
Ultraviolet light can cause damage to more than just your skin. It can damage the jacket of your coax, especially if you don’t buy good quality coax that has an outer jacket that is UV resistant. If the jacket degraded because of the UV light, it can allow water to get in.
- There is no significant difference between the two types
- RG-58 cable has less loss at a given frequency
- RG-8 cable has less loss at a given frequency
- RG-58 cable can handle higher power levels
RG-8 cable has less loss than RG-58 has. RG-58 has a very high line loss rating and should only be used for very short runs or in your vehicle. For any home installation, don’t use anything less than RG-8.
|Coax Cable Type, Size and Loss per 100 Feet|
|Coax Type||Size||Loss at HF 100 MHz||Loss at UHF 400 MHz|
|RG-58U||Small||4.3 dB||9.4 dB|
|RG-8X||Medium||3.7 dB||8.0 dB|
|RG-8U||Large||1.9 dB||4.1 dB|
|RG-213||Large||1.9 dB||4.5 dB|
|Hardline||Large, Rigid||0.5 dB||1.5 dB|
T9B11 Which of the following types of feed line has the lowest loss at VHF and UHF?
- 50-ohm flexible coax
- Multi-conductor unbalanced cable
- Air-insulated hard line
- 75-ohm flexible coax
The absolute best feed line when it comes to line loss is Air-Insulated Hard Line. The draw back to it is it takes special connectors and extra care to weather proof it to keep water from getting inside.
T7C02 Which of the following instruments can be used to determine if an antenna is resonant at the desired operating frequency?
- A VTVM
- An antenna analyzer
- A Q meter
- A frequency counter
An antenna analyzer is a great tool to have in your amateur radio tool bag. you can hook it directly to the bottom of an antenna or to the coax and test the SWR and Impedance of the antenna without having to get down from the roof or where ever you are.
T7C03 What, in general terms, is standing wave ratio (SWR)?
- A measure of how well a load is matched to a transmission line
- The ratio of high to low impedance in a feed line
- The transmitter efficiency ratio
- An indication of the quality of your station’s ground connection
The SWR are a measure of how well a load is matched to a transmission line. A 1:1 ratio is prefect, that means that everything your are transmitting is getting out the antenna at the other end, minus your line loss. If you have a 2:1 SWR, that means that only half of your signal is going out the antenna, the other half is being reflected back towards your radio.
T4A05 Where should an in-line SWR meter be connected to monitor the standing wave ratio of the station antenna system?
- In series with the feed line, between the transmitter and antenna
- In series with the station’s ground
- In parallel with the push-to-talk line and the antenna
- In series with the power supply cable, as close as possible to the radio
An SWR meter should be placed immediately after the radio and before the antenna system. the antenna system being, the antenna itself, antenna tuner and the coax itself.
T9B01 Why is it important to have a low SWR in an antenna system that uses coaxial cable feed line?
- To reduce television interference
- To allow the efficient transfer of power and reduce losses
- To prolong antenna life
- All of these choices are correct
The lower the SWR, the better the power transfer and the less loss you have.
T7C04 What reading on an SWR meter indicates a perfect impedance match between the antenna and the feed line?
- 2 to 1
- 1 to 3
- 1 to 1
- 10 to 1
The best or perfect SWR reading is a 1 to 1.
|*SWR Reading||Antenna Condition|
|4:1||Uh Oh! Something’s Wrong|
T7C05 What is the approximate SWR value above which the protection circuits in most solid-state transmitters begin to reduce transmitter power?
- 2 to 1
- 1 to 2
- 6 to 1
- 10 to 1
With most radios now a days, except for hand held radios, the power output will generally decrease if it detects an SWR reading higher than 2:1. this is to protect itself from damage.
T7C06 What does an SWR reading of 4:1 indicate?
- Loss of -4dB
- Good impedance match
- Gain of +4dB
- Impedance mismatch
A SWR of 4:1 means that something is deffinetly wrong somewhere. It means that there is an Impedance mismatch in your system that needs to be fixed ASAP before damage to your radio is done.
T9B09 What might cause erratic changes in SWR readings?
- The transmitter is being modulated
- A loose connection in an antenna or a feed line
- The transmitter is being over-modulated
- Interference from other stations is distorting your signal
Beyond bad tuning on the part of your antenna, a loose connection in your antenna or a feed line can also be to blame. Always check your connections before you mess with your antenna; It is the easiest thing to fix.
- Iambic pentameter
- Directional wattmeter
A directional wattmeter can also be used in place of a SWR meter. A directional wattmeter can be used to figure out your SWR by computing the power forward vs the power reversed.
T7C07 What happens to power lost in a feed line?
- It increases the SWR
- It comes back into your transmitter and could cause damage
- It is converted into heat
- It can cause distortion of your signal
Power that is lost in a feedline is converted into heat.
T9B04 What does an antenna tuner do?
- It matches the antenna system impedance to the transceiver’s output impedance
- It helps a receiver automatically tune in weak stations
- It allows an antenna to be used on both transmit and receive
- It automatically selects the proper antenna for the frequency band being used
An antenna tuner basically tricks the radio into thinking that it has a good match with the antenna system allowing for maximum power to be transmitted out of the radio. To read more about antenna tuners, check out this post.
- To prevent the radiation of signals when making tests
- To prevent over-modulation of your transmitter
- To improve the radiation from your antenna
- To improve the signal to noise ratio of your receiver
A dummy load is used to check your radio signal without the transmission being sent out over the airwaves.
T7C13 What does a dummy load consist of?
- A high-gain amplifier and a TR switch
- A non-inductive resistor and a heat sink
- A low voltage power supply and a DC relay
- A 50 ohm reactance used to terminate a transmission line
A dummy load consists of a non-inductive resistor and a heat sink. A commercial dummy load can be expensive but you can build them yourself from a large cluster of small carbon resistors. You can generally find a used dummy load at a hamradio swap meet.
So that brings us to the end of this section. Next week we will be talking about Safety First! Please share my blog with your friends and if you have not done so already, please subscribe to my email list to get emails on when I publish a new post. Please Like me on Facebook, and follow me on Twitter, Google+, LinkedIn and StumbledUpon. Links to all of these can be found under social on the menu.
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Until next time…
73 de Curtis, K5CLM
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