Cheap easy homebrew antenna WORKS
- Thread starter nashvillebill
- Start date
"Antennas are mighty finicky and I'm no expert, but adding 140 pf of capacitance doesn't seem good."
The idea that capacitance on the feedline is bad has no relevence at all. The transmission line equation which calculates the input impedence from the load impedence has no parameter for feedline capacitance. However, It does use the impedence of the transmission line as a parameter.
All 2 wire transmission lines have a certain amount of capacitance/foot between the wires, as well as inductance/foot.
Perhaps you're thinking about the ramifications of phono input cable and high capacitance of the cable having an impact on the signal. This is because of the very high input impedence of the phono input. And only when you're around 1000 pF total capacitance is this going to be noticeable.
Antenna feedline capacitance or inductance has no bearing at all on the signal received.
The idea that capacitance on the feedline is bad has no relevence at all. The transmission line equation which calculates the input impedence from the load impedence has no parameter for feedline capacitance. However, It does use the impedence of the transmission line as a parameter.
All 2 wire transmission lines have a certain amount of capacitance/foot between the wires, as well as inductance/foot.
Perhaps you're thinking about the ramifications of phono input cable and high capacitance of the cable having an impact on the signal. This is because of the very high input impedence of the phono input. And only when you're around 1000 pF total capacitance is this going to be noticeable.
Antenna feedline capacitance or inductance has no bearing at all on the signal received.
dr*audio
Fish fingers and custard!
Right, as long as the impedance of the transmission line matches the system impedance it will work. The insertion loss of the cable does matter, though.
electroking
a- v- karma member
a tiny bit of transmission line theory...
Note however that the transmission line impedance is related to the
distributed parameters as follows:
line impedance = square root (distributed inductance/distributed capacitance).
Note however that the transmission line impedance is related to the
distributed parameters as follows:
line impedance = square root (distributed inductance/distributed capacitance).
The capacitance between conductors in Cat 5 cable is a concern for transmitting data over distance and will effect the bandwidth. When he twisted the ends of the Cat 5 together he effectively made it one multistrand conductor and the capacitance was shorted. The capacitance measurement of the cable is meaningless.
I've read some 'net articles saying this loop's impedance is 300, others say 100..
The folded dipole is a 1 wavelenth loop of wire that has been stretched out. It's impedence is close to 300 ohms. A 1 wavelength square or delta loop has an impedence of somewhere around 100 to 140 ohms.
So the above statement is true in both cases, just know which kind of loop one is talking about.
The folded dipole is a 1 wavelenth loop of wire that has been stretched out. It's impedence is close to 300 ohms. A 1 wavelength square or delta loop has an impedence of somewhere around 100 to 140 ohms.
So the above statement is true in both cases, just know which kind of loop one is talking about.
electroking
a- v- karma member
With all due respect, RF transmission lines are characterized by
their distributed capacitance and inductance, and although users
don't need very much to care about those things, these parameters
are far from meaningless. They have practical consequences, for instance,
a short at the far end of a cable that is a quarter-wavelength long
is seen at the input of the cable as an open circuit.
Please do study some adequate material before making erroneous
statements. A good starting point would be the ARRL Handbook.
Regards to all.
electroking
a- v- karma member
If you're discussing my credentials, I am a holder of the former
amateur, advanced, and digital Canadian Amateur Radio certificates,
I was tested for Morse Code at 15 WPM and I also hold a Ph.D.
in electrical engineering with a specialty in microwave circuits.
Let's have a discussion about any problem in radio technology anyday!
73 de VE2GTP (ham since 1982)
P.S. : stranded antenna wire is fine...
amateur, advanced, and digital Canadian Amateur Radio certificates,
I was tested for Morse Code at 15 WPM and I also hold a Ph.D.
in electrical engineering with a specialty in microwave circuits.
Let's have a discussion about any problem in radio technology anyday!
73 de VE2GTP (ham since 1982)
P.S. : stranded antenna wire is fine...
electroking
a- v- karma member
:scratch2:
I believe this thread has reached the end of its useful life.
Regards to all, apologies to whomever I may have hurt.
I believe this thread has reached the end of its useful life.
Regards to all, apologies to whomever I may have hurt.
Nonsense. If you felt my comment was about using the cat 5 cable as some kind of feedline then you were correct. He may have been for all I know. I just thought he was talking about twisting the end on some leftovers rather than going out to buy some new single conductor to make an antenna.
Yep.
It was easier ( and less wasteful) to use one length of Cat 5 and connect the conductors than to rip out a single light strand. All I'm looking to do is get some better reception on some NPR and college FM stations here, and if I can hang a square on my wall, or even in the attic as opposed to stick a mast and whatever up on the roof, I'm all for it :yes:
Endspec
Super Member
All I know is, it is working, I made one and don't even have it totaly square on the wall and my meter went up from 2.5 to 4.5 on the local collidge station and alot of the standerd hiss from them went away, next is to make a fram for it on a stand to adjust it for the other stations.
This antenna is cut to a specific wave length, but presumably used for the whole "broad" FM band. In this case, the larger wire size will have broader bandwidth, meaning that it will have better signal in both ends of FM band. So using all strands of CAT5 cable is actually better than a thin wire. You can get similar effect using coax cable as antenna element.
The dielectric constant of the cable insulation (including environment) does have an effect on effective wave speed, and hence wave length and antenna size. But in FM band, this effect is small and the antenna is supposed to be "broadband", so it is not critical.
The dielectric constant of the cable insulation (including environment) does have an effect on effective wave speed, and hence wave length and antenna size. But in FM band, this effect is small and the antenna is supposed to be "broadband", so it is not critical.
dr*audio
Fish fingers and custard!
I just got finished installing one at home, in the living room, where I couldn't get easy access to the attic antenna. Just taped it to the wall in the diamond shape, with the tail at the bottom, feeding a balun, going to coax to the tuner. Works great! I had lots of multipath problems and noise in that room before. All gone now.:thmbsp:
stuartk
Super Member
I'm not sure you need a balun. http://www.bloomington.in.us/~wh2t/loop.html
The impedance is evidently about 100 ohms, so 75 ohm coax is a reasonably close match.
dr*audio
Fish fingers and custard!
Without a balun, you will get reflections back up the antenna due to mismatch. 100 ohms to 75 ohms isn't close enough. If you use twin lead you will pick up interference and multipath through the twin lead. Invariably I have gotten much better reception with any antenna if I use a balun and 75 ohm coax, rather than twin lead, even over a 6 foot length.
stuartk
Super Member
I guess I'll have to try it both ways and see what happens.