User Panel
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?
|
|
If its broke. I can fix it. If it doesn't exist. I can make it. If its metal.
|
[#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.
|
|
World ain't what it seems, is it Gunny?
|
[#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 |
|
Sic semper tyrannis!
|
[#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.
|
|
N&MEM, SSDR, NRA Life Member
Gun control is literally Hitler. |
[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. |
|
This is...a clue - Pat_Rogers
I'm not adequately aluminumized for this thread. - gonzo_beyondo CO, MI, OR - Please lobby your legislators to end discrimination against non-resident CCW permit holders |
[#5]
Originally Posted By SteveOak: The length has nothing to do with it. View Quote 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. |
|
|
[Last Edit: SteveOak]
[#6]
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. View Quote 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. |
|
N&MEM, SSDR, NRA Life Member
Gun control is literally Hitler. |
[Last Edit: Gamma762]
[#7]
Originally 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. View Quote View All Quotes View All Quotes Originally 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. |
|
This is...a clue - Pat_Rogers
I'm not adequately aluminumized for this thread. - gonzo_beyondo CO, MI, OR - Please lobby your legislators to end discrimination against non-resident CCW permit holders |
[#8]
Thank you, Gamma!
|
|
|
[#9]
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. 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. View Quote Thank you for saying I was correct. |
|
N&MEM, SSDR, NRA Life Member
Gun control is literally Hitler. |
[#10]
Try this one on for size! ARRL article,"My Feed Line Tunes My Antenna" 73, Rob
|
|
Sic semper tyrannis!
|
[#11]
Originally Posted By SteveOak: Thank you for saying I was correct. View Quote View All Quotes View All Quotes 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. |
|
|
[#12]
Whatever. LOL
|
|
N&MEM, SSDR, NRA Life Member
Gun control is literally Hitler. |
[#13]
Impedance, length, and velocity factor all come into play.
|
|
|
[#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.
|
|
If its broke. I can fix it. If it doesn't exist. I can make it. If its metal.
|
[#15]
Originally 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. View Quote 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. |
|
This is...a clue - Pat_Rogers
I'm not adequately aluminumized for this thread. - gonzo_beyondo CO, MI, OR - Please lobby your legislators to end discrimination against non-resident CCW permit holders |
[#16]
No worries on the lost length!,........the water intrusion makes up for it.
|
|
|
[#17]
|
|
If its broke. I can fix it. If it doesn't exist. I can make it. If its metal.
|
[Last Edit: Positronic]
[#18]
Originally Posted By SteveOak: 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. View Quote Usually, if a simple impedance match is desired, an electrical quarter wavelength is used. If it is a PHASE delay line, for azimuth control of a directional multi-element array, a different fractional wavelength is used. on HF bands an inch or three won't make much difference. |
|
|
Sign up for the ARFCOM weekly newsletter and be entered to win a free ARFCOM membership. One new winner* is announced every week!
You will receive an email every Friday morning featuring the latest chatter from the hottest topics, breaking news surrounding legislation, as well as exclusive deals only available to ARFCOM email subscribers.
AR15.COM is the world's largest firearm community and is a gathering place for firearm enthusiasts of all types.
From hunters and military members, to competition shooters and general firearm enthusiasts, we welcome anyone who values and respects the way of the firearm.
Subscribe to our monthly Newsletter to receive firearm news, product discounts from your favorite Industry Partners, and more.
Copyright © 1996-2024 AR15.COM LLC. All Rights Reserved.
Any use of this content without express written consent is prohibited.
AR15.Com reserves the right to overwrite or replace any affiliate, commercial, or monetizable links, posted by users, with our own.