23
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How does an SWR meter really work?
Dispelling the Myth
To begin with, the typical HF SWR meter does not have the ability to separately sample the forward and reverse power, voltage, or current. Any description of the device or its ...
- 18.3k
13
votes
I have 60 feet of coax but only need 20 feet; can I loop up the excess?
There's nothing inherently wrong with “looping up” extra coaxial cable. In fact, a neatly wound coil of coax can function as an air-core choke balun (“ugly balun”) which is useful for some antenna ...
- 23.7k
10
votes
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Why is my calculation for added length of coax for a double cross antenna different to everyone else's?
The difference is the "Velocity factor". A 36cm long physical coax wire of this type is electrical 54.4cm long.
Different types of wire have different velocity factors.
https://en.wikipedia.org/wiki/...
- 126
9
votes
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Is "low loss" cable required for HF?
Your question seems as much about psychology as much as technical concerns. We mainly favor the technical questions, but I'll take a stab at the psychological aspects also. All of you, please feel ...
- 8,982
8
votes
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Why isn't twisted pair used for feedlines?
Balanced lines (of which twisted pair is a special type) really have an upper frequency limit; you can't use them to transport 1 GHz (well, you can, but the smallest variation in direction or distance ...
- 14.7k
8
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Does a balun need to be made with coax?
I'll explain the operation of that balun very briefly: for the differential mode (which by definition has equal but opposite currents on each conductor), each conductor induces an equal but opposite ...
- 50.8k
8
votes
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Soldering PL-259.. with a torch?
Our friends in the UK and other parts of the world are now wondering how you could even begin to solder a PL259 connector with a torch (aka flashlight)! But in their vernacular, you of course are ...
- 18.3k
8
votes
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VHF 50 Ω Antenna Over 75 Ω TV Coax
I will need a UHF/VHF diplexer on either end to suitably merge/split the signals from each antenna
Yes, this is correct.
A tangent: If you wanted to save some money by using mass-market parts, ...
- 23.7k
8
votes
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50Ohm Coax presents 50 Ohms of impedance on which frequency?
For practical purposes, all frequencies.
If you cut a transmission line into infinitesimal segments, each segment can be modeled as:
simulate this circuit – Schematic created using CircuitLab
($...
- 50.8k
7
votes
How is a 1:1 current balun different from a choke?
A choke is an inductor which is used to block high frequencies while allowing DC to pass. All chokes are inductors (though sometimes more than one inductor), but not all inductors are chokes: to be ...
- 50.8k
7
votes
Coax cable as antenna for 5 watt handy
The electrically shortened antennas often found on HTs are not simple pieces of wire, but coils:
By Shootthedevgru at English Wikipedia, CC BY-SA 3.0
The coil adds inductance over the length of the ...
- 50.8k
7
votes
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It it really useful to terminate unused 75Ω outputs on a coax splitter?
Terminating unused ports will never make things worse, and indeed is necessary to provide "ideal" behavior. Is ideal behavior really necessary? It depends on the application. Is your setup working now?...
- 50.8k
7
votes
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Can ferrite beads lower coax cable velocity factor?
No, adding ferrite beads to choke currents on the outside of the shield of a coaxial cable does not affect its impedance or velocity factor. Impedance and velocity factor are determined by the inside ...
- 7,743
7
votes
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Trying to understand the work of 4:1 coax balun
Think of it first without the feedline:
The half-wavelength of transmission line provides a 180 degree phase shift, and the (short) connection between the shield ends is "ground". That means it's at ...
- 50.8k
7
votes
What does an antenna analyzer tell me about these coaxial cables?
My name is Alex, I'm the head of technical support at RigExpert.
This is actually an interesting question.
Our engineers could not give an exact answer why the schedule in the second case behaves ...
6
votes
Accepted
How is a 1:1 current balun different from a choke?
Baluns are designed to be transformers (like 1:1 4:1, 6:1, etc.) or choke baluns, and both.
For an antenna, the purpose of a choke balun is to create a high-impedance to common mode currents that ...
- 2,898
6
votes
Can a common-mode current exist on the inside of a coax shield?
It is helpful to understand the basic functioning of a coaxial cable. But first there are two important phenomenons that must be understood in order to proceed.
Skin Effect
When direct current (time ...
- 18.3k
6
votes
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Does Coax Limit the Maximum SWR a Transmitter 'Sees'?
The source of the SWR limit on the transmitter end is the losses in the feedline. In general, the higher the matched line loss, the lower the maximum SWR that will be present at the transmitter end. ...
- 18.3k
6
votes
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Suspected Damage in Coax Cable
It appears to be typical moisture ingress. The joint between the coax and the connector is not inherently waterproof. A damaged jacket can also be the source of moisture ingress. Normally the ...
- 18.3k
6
votes
Accepted
Why does ladder line generally have a lower loss than coaxial cable?
When the SWR is 1:1, the matched line loss of ordinary ladder line is lower than the matched line loss of ordinary coax because at HF, most of the loss is $I^2R$ loss, and the current magnitude is ...
- 1,646
6
votes
Can ferrite beads lower coax cable velocity factor?
For well-designed coax, the EM fields are confined to the space between the inside of the braid and the center conductor, i.e. the dielectric insulation region which affects the velocity factor. ...
- 1,646
6
votes
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Recomended coax for 100ft (2m/70cm)
According to this online coax loss calculator, the loss of 50 feet of new Belden 9258 RG-8X with an antenna with an SWR of 1.5 is 4.29 dB at 450 MHz (UHF), so the loss for 100 feet of the same coax ...
- 8,982
6
votes
One cable two different radios
You need a switch. Connecting two transceivers directly to one antenna is a great way to destroy one of them — as soon as you transmit with one, the receive circuit of the other one will see several ...
- 10.7k
6
votes
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How to calculate coax cable loss for lengths other than 100 feet
It's the first one, if you use 50 feet (1/2 the length) then the loss is 20 dB / 2 = 10 dB.
similarly, if you used 200 feet, the loss would be double… 20dB + 20dB = 40dB loss.
there are several online ...
- 3,044
5
votes
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RG-174 to pull signal from 1st IF?
So, you say:
I don't care if the signals are delayed. I'm just tapping the line out to a pan adapter. I am most concerned about influencing the existing function of the receiver
And that's exactly ...
- 14.7k
5
votes
How is a 1:1 current balun different from a choke?
The name choke refers to the electrical component, whereas the name 1:1 current balun refers to the job it is doing.
There's more than one way to construct a balun.
There are purposes for a choke ...
- 23.7k
5
votes
Accepted
Understanding coax radiation and "current on the shield"
Perhaps some of my confusion comes from an audio background, where one has "balanced" and "unbalanced" cabling in a way that is easier for me to understand: an unbalanced cable is simply a signal and ...
- 50.8k
5
votes
Does a balun need to be made with coax?
Short answer: No, and in fact you'll often get more bang for your buck by avoiding coaxial baluns.
The description of the balun's purpose in an antenna system you quote is an oft repeated bit of "...
- 1,824
5
votes
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Use a TV coax and balun for a poor-man's OCFD?
OK, let me try to answer this, but this answer may also be qualified as unqualified.
If you have a 50 Ohm receiver, and connect a perfectly (Z=R) 75 ohm antenna system, then your VSWR would be 1.5, ...
- 2,107
5
votes
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Why is the insertion loss of my coax cable higher than expected?
Based on your description, I would suspect moisture damage.
Moisture ingress in coax cables typically results in the corrosion of the copper braided wires in the shield. The oxide that forms ...
- 18.3k
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