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Posted: 4/22/2024 8:20:51 PM EDT
My Butternut HF6V antenna has a 75 ohm matching cable. Last week I disconnected my R8 coax and moved it aside to mow the yard. I'm still using a spare bedroom in the house as my temporary shack so the coax just lays on the ground. It will be buried when my real shack is completed. When I went to reconnect the coax the connector on the end of the matching cable broke. I ordered a new connector and installed it Sunday. I lost less than half an inch of length. It all works fine. SWR is the same. What if the matching cable was damaged in the middle. Could I solder on a new connector and be good to go? My question is. It the length of a matching cable important? Or just that it's 75 ohms?
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Posted: 4/22/2024 8:25:51 PM EDT
[#1]
That short amount won't make much difference, but there is a formula for the matching cable. I will go see if I can find it.
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Posted: 4/22/2024 9:12:15 PM EDT
[#2]
The answer is "both", the surge impedance of 75 Ohms and the length of the cable. We see the same thing in adjusting the length of ladder line to tune an antenna. For a test oif your antenna, check what frequency the advertised 2:1 or 3: 1 SWR points are. It may have changed or not. I built a 4:1 coaxial cable BALUN for 6 meters according to the hand book. I was kinda amazed it worked. hth
73, Rob |
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Posted: 4/23/2024 8:58:41 PM EDT
[#3]
The 75 ohms is from the center conductor to the shield. It is a function of the distance from the center conductor to the shield and the characteristic of the dielectric. The length has nothing to do with it.
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Posted: 4/23/2024 9:20:14 PM EDT
[Last Edit: Gamma762]
[#4]
Yes the length of the cable is important along with the impedance.
On that particular one, it's supposed to be a 1/4 wavelength at 20 meters. I've been dealing with this myself as I've been rebuilding one of those antennas and it didn't come with the matching cable. Half an inch shouldn't be a problem at that frequency, but a foot would be. Half definitely would be. |
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Posted: 4/23/2024 9:26:56 PM EDT
[#5]
Quote HistoryOriginally Posted By SteveOak: The length has nothing to do with it. Length is extremely important. It must be a quarter wave and that must take into account the velocity of propagation of the coax. DX Engineering sells a ready made replacement for this part. |
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Posted: 4/23/2024 10:02:06 PM EDT
[Last Edit: SteveOak]
[#6]
Quote HistoryOriginally Posted By aa777888-2: Negative Ghost Rider! Length is extremely important. It must be a quarter wave and that must take into account the velocity of propagation of the coax. DX Engineering sells a ready made replacement for this part. My statements were regarding characteristic impedance and are absolutely correct. 75 ohm cable is 75 ohms whether it is 1 meter or 50 feet long. |
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Posted: 4/24/2024 12:30:54 AM EDT
[Last Edit: Gamma762]
[#7]
Quote HistoryOriginally Posted By SteveOak: My statements were regarding characteristic impedance and are absolutely correct. 75 ohm cable is 75 ohms whether it is 1 meter or 50 feet long. Quote HistoryOriginally Posted By SteveOak: Originally Posted By aa777888-2: Negative Ghost Rider! Length is extremely important. It must be a quarter wave and that must take into account the velocity of propagation of the coax. DX Engineering sells a ready made replacement for this part. My statements were regarding characteristic impedance and are absolutely correct. 75 ohm cable is 75 ohms whether it is 1 meter or 50 feet long. True, but it's function as an impedance matching device (such as OP's case) uses that impedance in a length-dependent way. It's a 75 ohm transmission line of x wavelengths at y frequency so has a particular effect. As a 1/4 wave on 20 meters as an example, it would take a higher impedance of a little over 100 ohms (112.5), and "flip" it around the 75 ohm impedance of the line down to 50 ohms. A 1/2 wavelength is impedance-transparent... so if a 75 ohm line is 1/2 wavelength or multiple thereof, a 50 ohm antenna as an example would appear as 50 ohms at the other end of the line, ignoring the 75 ohm impedance of the line. This is how people often use 75 ohm cable in 50 ohm systems, by tuning the line length. |
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Posted: 4/24/2024 7:55:14 AM EDT
[#8]
Thank you, Gamma!
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Posted: 4/24/2024 7:51:07 PM EDT
[#9]
Quote HistoryOriginally Posted By Gamma762: True, but it's function as an impedance matching device (such as OP's case) uses that impedance in a length-dependent way. It's a 75 ohm transmission line of x wavelengths at y frequency so has a particular effect. As a 1/4 wave on 20 meters as an example, it would take a higher impedance of a little over 100 ohms (112.5), and "flip" it around the 75 ohm impedance of the line down to 50 ohms. A 1/2 wavelength is impedance-transparent... so if a 75 ohm line is 1/2 wavelength or multiple thereof, a 50 ohm antenna as an example would appear as 50 ohms at the other end of the line, ignoring the 75 ohm impedance of the line. This is how people often use 75 ohm cable in 50 ohm systems, by tuning the line length. Thank you for saying I was correct. |
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Posted: 4/24/2024 8:04:41 PM EDT
[#10]
Try this one on for size! ARRL article,"My Feed Line Tunes My Antenna" 73, Rob
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Posted: 4/24/2024 8:40:15 PM EDT
[#11]
Quote HistoryOriginally Posted By SteveOak: Thank you for saying I was correct. Quote HistoryOriginally Posted By SteveOak: Thank you for saying I was correct.  OP writes: Originally Posted By Ohio_Sharkman: My question is. It the length of a matching cable important? Or just that it's 75 ohms? Originally Posted By SteveOak: The length has nothing to do with it. Originally Posted By Gamma762: True, but it's function as an impedance matching device (such as OP's case) uses that impedance in a length-dependent way.  |
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Posted: 4/25/2024 9:21:25 AM EDT
[#12]
Whatever. LOL
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AZ, USA
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Posted: 4/25/2024 9:47:55 AM EDT
[#13]
Impedance, length, and velocity factor all come into play.
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Posted: 4/26/2024 6:44:31 PM EDT
[#14]
Thanks for the reply's. The matching cable works, after my repair. DXE does sell an exact replacement for $80. I really don't like the connector on the matching cable. There are no threads, or any other mechanical means of keeping the inner and outer conductors from moving independently. The heat shrink tubing is the only thing keeping the connector from twisting and breaking the center conductor. Once it's sealed and underground it will probably last a long time.
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Posted: 4/26/2024 7:15:22 PM EDT
[#15]
Quote HistoryOriginally Posted By Ohio_Sharkman: Thanks for the reply's. The matching cable works, after my repair. DXE does sell an exact replacement for $80. I really don't like the connector on the matching cable. There are no threads, or any other mechanical means of keeping the inner and outer conductors from moving independently. The heat shrink tubing is the only thing keeping the connector from twisting and breaking the center conductor. Once it's sealed and underground it will probably last a long time. I contemplated putting a BNC on the antenna end of mine, with a BNC to Banana adapter. And then a UHF female on the other end to connect to the 50 ohm feedline. RG59 to make it easier to include a choke as I don't plan on using an amplifier. |
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USA
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Posted: 4/28/2024 10:55:57 AM EDT
[#16]
No worries on the lost length!,........the water intrusion makes up for it.
 
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Posted: 4/28/2024 8:09:16 PM EDT
[#17]
Quote HistoryOriginally Posted By SteelonSteel: No worries on the lost length!,........the water intrusion makes up for it. It's not ideal, but I do protect the connector from water. The end connected to the antenna is sealed very well. |
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