Amateur Radio Stack Exchange
Amateur Radio Stack Exchange

Questions tagged [transmission-line]

70 questions
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How does an SWR meter really work?

For ham radio operators, the SWR meter is a ubiquitous piece of equipment. There are dozens of standalone models on the market and most modern transceivers and antenna tuners have this functionality built in. Any ham with an interest in antennas has…
Glenn W9IQ
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Does Coax Limit the Maximum SWR a Transmitter 'Sees'?

A recent discussion about SWR led to the assertion that a length of coax cable will ultimately limit the SWR to which the transmitter will be exposed. By experimenting with an SWR calculation tool like TLDetails, there appears to be a basis for this…
Glenn W9IQ
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Is there an optimum transmission line length for maximum power transfer?

Consider a typical source -> transmission line -> load (i.e. TX->transmission line->antenna) arrangement and assume that the load is not matched to the transmission line characteristic impedance. It's always possible to select a length of…
Chu
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How do I design a round or square coaxial transmission line to have a specific impedance?

Certain Hex Beam antennas use a coaxial center support post to send the RF to each antenna section, which is superior, in terms of maintenance, to a post with screw terminals connected by short lengths of coax used in previous designs. I'd like to…
Adam Davis
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Attic Dipole - Impact of Ethernet over CAT5/6

I've been thinking about putting 20m and 10m dipoles up in my attic (HOA), but I also intend to soon put some Ethernet cable to a WAP (with PoE) through the attic. I was intending to feed this with CAT5e cable because I have lots of it and it's…
William
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Why isn't twisted pair used for feedlines?

My son was asking about some Ethernet cable (Cat 5 unshielded twisted pair), and since he recently passed his Technician exam and we both have "radio stuff" on our minds, I tried to make the analogy between coax transmission lines and the balanced…
natevw - AF7TB
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Will there be common-mode currents if a coax followed by a ladder line is terminated in a dummy load?

Assuming an antenna system where a transmitter feeds an ideal coaxial cable which is then connected to an ideal ladder line without a balun: It is often said that common-mode currents appear in the transition between a coaxial cable and a ladder…
rubund
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Importance of Ferrite Size in Coax Choke Balun

The question is quite simple: When making a coax choke balun, does the size of the ferrite matter, and if so, why? I have seen design notes that say to use a single FT240-43 toroid for powers up to 400W, and then to use two stacked FT240-43 toroids…
M1GEO
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Why does ladder line generally have a lower loss than coaxial cable?

It is well understood in the amateur radio community that ladder line, window line, etc. have lower losses per foot than say RG-213 or LMR400, particularly on HF frequencies. It is also understood that this loss is a function of the RF resistance of…
Glenn W9IQ
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SWR Measured at the Transmitter versus SWR at the Antenna

It is fairly well understood that due to transmission line losses, the SWR as measured at the transmitter will be less than the SWR at the antenna. Some of the new antenna analyzers and some vector network analyzers can 'zero out' or compensate for…
Glenn W9IQ
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How can a quarter-wave transmission line transformer be implemented with lumped elements?

Sometimes designs call for quarter-wave sections of transmission lines. There are some situations when using actual transmission lines would not be practical: The necessary characteristic impedance is something not easily obtained A quarter…
Phil Frost - W8II
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What happens in a low loss feed-line with a high SWR

I was recently reading an interesting article from the ARRL to get a better understanding of the SWR. The following case is still unclear: A low loss feed-line with an impedance that doesn't match the antenna's impedance and the receiver's…
ITChap
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Can a common-mode current exist on the inside of a coax shield?

People often say common-mode current flows on the outside of the shield, while the current on the inside of the shield is always opposed by an equal but opposite current on the center conductor. This is because skin effect separates the inside and…
Phil Frost - W8II
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Understanding coax radiation and "current on the shield"

I'm having trouble understanding what seems to be the fundamental principle behind baluns (and probably other related topics). Perhaps some of my confusion comes from an audio background, where one has "balanced" and "unbalanced" cabling in a way…
natevw - AF7TB
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What exactly does an unun do to the impedance of a transmission line?

If I have a load Z_L and a length of transmission line with impedance Z_0 I know how to compute the impedance at the input to the transmission line (using smith chart or using telegraph equation). But now suppose I attach a 4:1 Unun to the (input)…
DrEntropy
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