24 votes
Accepted

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 ...
Glenn W9IQ's user avatar
  • 18.6k
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 ...
Kevin Reid AG6YO's user avatar
  • 24.5k
10 votes
Accepted

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/...
Joerg's user avatar
  • 126
10 votes
Accepted

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 ...
rclocher3's user avatar
  • 9,182
9 votes
Accepted

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 ...
Marcus Müller's user avatar
9 votes
Accepted

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 ($...
Phil Frost - W8II's user avatar
9 votes
Accepted

Does a halfwave (or multiple) length of coax duplicate the load impedance of one end at the other end?

I have been told that 50 ohm coax is 50 ohm at both ends, and 50 ohms all along its length, and it is 50 ohms that is presented to anything to which it is connected, and the length of it does not ...
hobbs - KC2G's user avatar
  • 12.2k
8 votes
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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 ...
Phil Frost - W8II's user avatar
8 votes
Accepted

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?...
Phil Frost - W8II's user avatar
8 votes
Accepted

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 ...
Glenn W9IQ's user avatar
  • 18.6k
8 votes
Accepted

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, ...
Kevin Reid AG6YO's user avatar
  • 24.5k
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 ...
Phil Frost - W8II's user avatar
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 ...
Phil Frost - W8II's user avatar
7 votes
Accepted

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 ...
Brian K1LI's user avatar
  • 7,828
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 ...
Phil Frost - W8II's user avatar
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 ...
Alexander Antonov's user avatar
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 ...
Glenn W9IQ's user avatar
  • 18.6k
6 votes
Accepted

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. ...
Glenn W9IQ's user avatar
  • 18.6k
6 votes
Accepted

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 ...
Glenn W9IQ's user avatar
  • 18.6k
6 votes
Accepted

Testing for 'Bad' Coax

You're on the right track with the first items in your list. Assuming that you have a wattmeter and a dummy load that both match the impedance of the coax, it's a simple matter to measure the loss. ...
Mike Waters's user avatar
  • 7,919
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 ...
Cecil - W5DXP's user avatar
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. ...
Cecil - W5DXP's user avatar
6 votes

Using 75 Ω instead of 50 Ω coax feed

Sure, you can use 75 ohm coax instead of 50 ohm. In some cases (such as feeding a dipole), 75 ohm coax may be a better match to the load than 50 ohm. In other cases (such as feeding a vertical), it ...
Phil Frost - W8II's user avatar
6 votes
Accepted

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 ...
rclocher3's user avatar
  • 9,182
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 ...
hobbs - KC2G's user avatar
  • 12.2k
6 votes
Accepted

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 ...
webmarc's user avatar
  • 4,038
6 votes

What kind of soldering iron is best for PL-259 (UHF) connectors?

I'm going to answer a slightly different question to the one asked... I promise not to be offended if it's ignored or even downvoted (if it receives enough downvotes, I'll delete): Crimping, with the ...
webmarc's user avatar
  • 4,038
6 votes

Does Velocity Factor pose a significant cable loss?

Velocity factor is not the cause of cable loss, but it is related. Signals in a coaxial cable travel as transverse electromagnetic waves with currents in the copper and electric fields in the ...
tomnexus's user avatar
  • 10.9k
6 votes

Ohms & coax cabling miscalculation?

First: if your multimeter reads 0.7 ohms with the probes shorted, it's not very trustworthy, and the rest of the calculation should be ignored. (Try putting a new battery in). The correct DCR for RG6 ...
hobbs - KC2G's user avatar
  • 12.2k
6 votes
Accepted

Using UV5R HTs. Will i lose receiving range by attaching coaxial cable to put my antenna remotely as well as higher?

The answer is that it depends, although in my experience increasing height will always improve range. Height matters a lot in most VHF propagation. For your UV5R HT, antenna height will generally ...
Deepstop's user avatar
  • 246

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