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Message |
    
Charliebrown
Intermediate Member
Username: Charliebrown
Post Number: 284
Registered: 10-2002
| | Posted on Tuesday, September 24, 2013 - 9:49 am: | 
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I was wondering that if I could take two of these 750 tv line's put them side by side and use the center of each line, one for the center connection for the radio and to the antenna and the other center of the other line for the shield and in doing so would it decrease my line loss for this tv cable. If you remember this line is 75 ohm and is a semi flex aluminum shield. It is what the tv company use for their main line. |
Tech833
Moderator
Username: Tech833
Post Number: 2268
Registered: 8-2002
| | Posted on Thursday, September 26, 2013 - 8:45 am: |
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Hi CB.
I don't exactly understand the question. Are you takling about using two runs of cable together as a shielded twinlead? Confused.
Your radio 'Mythbuster' since 1998
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Charliebrown
Intermediate Member
Username: Charliebrown
Post Number: 285
Registered: 10-2002
| | Posted on Thursday, September 26, 2013 - 10:15 am: |
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I hope I can explain better. I want to run the two line's together side by side. Then I want to use the center line of the first cable as you would any coax for the radio to antenna hook up. Then instead of using the shield on either cable, I want to use the center wire on the second cable for the shield. If I do this will this make the cable have less loss? My thinking on this is that perhaps it would be like 600 ohm ladder line. The more distance between the two line's the less line loss. The reason I am still using the 750 tv line is that I cannot afford the best cable. |
Tech237
Moderator
Username: Tech237
Post Number: 1699
Registered: 4-2004
| | Posted on Saturday, September 28, 2013 - 9:52 pm: |
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No. You can use 75 ohm coax on it's one, with minimum issues if all else is up to spec.
I have used 75 semi-flex coax on radio/antenna links form HF to 70cm (hey I could get it cheap $0 then). Worse case was an SWR of 1.5:1.
Doing what you suggested would not work, and definitely would not lower losses. It would create as 833 said a twin-lead with each conductor individually sheilded. I have used that method a time or two, as well. When I do I ground the shack end of the shields, but leave the atenna end open.
Hope this helps a little.
Tech237
N7AUS
God made me an athiest, who are you to question his wisdom?
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Charliebrown
Intermediate Member
Username: Charliebrown
Post Number: 286
Registered: 10-2002
| | Posted on Sunday, September 29, 2013 - 7:15 am: |
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Thanks for the information. |
Tech833
Moderator
Username: Tech833
Post Number: 2270
Registered: 8-2002
| | Posted on Sunday, September 29, 2013 - 11:43 am: |
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I think I understand your question. If my understanding is correct, then I agree with Simon for your answer.
Your radio 'Mythbuster' since 1998
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Charliebrown
Intermediate Member
Username: Charliebrown
Post Number: 287
Registered: 10-2002
| | Posted on Monday, September 30, 2013 - 1:19 pm: |
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One reason I had ask this question is that I had thought that as it is with 600 ohm ladder line or open wire. The more space between the center wire's the lower the loss. Up to a point. If you remember I have been in the radio hobby for a long time but there is alway's a lot more any person can alway's learn. Keep teaching me tech's and we will learn. Time's are hard here where I live and the tv line is all I can afford for now. It work's great. I have a 1 to a 1.1 swr on 500 foot of line with an I-max 2000. Thank's a lot for the time and the help you have given. |
Tech237
Moderator
Username: Tech237
Post Number: 1702
Registered: 4-2004
| | Posted on Tuesday, October 01, 2013 - 6:14 pm: |
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IT is BUT, you cannot use that sort of feed unless the antenna is designed for it.The line line style of feedline is a BALANCED feed (equal current on both lines - if all is set up correctly), while most modern CB, Ham radio are designed for an UNBALANCED feed.
Feeding antennas is not complex, but it isn't the simplest thing in the world either. See if you can find an "older" Antenna Handbook (ARRL, RSGB or even Bill Orr's) they have some great information on twinlead and coax feed systems.
Tech237
N7AUS
God made me an athiest, who are you to question his wisdom?
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Charliebrown
Intermediate Member
Username: Charliebrown
Post Number: 288
Registered: 10-2002
| | Posted on Wednesday, October 02, 2013 - 7:03 am: |
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Thank you 237 for your quick response. You are right it is not simple and that is why I ask. So, let me ask this--In running this cable as I had said in my question before, what would be the benefit if any in doing this. 237 , since you had done this already I am interested in the detail's. Since this is the only cable I have and can afford I had been wanting to get the benefit of twin lead out of the 75 ohm semiflex. I understand that I will not but, would there be any benefit. Remember , I would like to have some detail's on how it worked for you. Thank you for your help and time. |
Scrapiron63
Advanced Member
Username: Scrapiron63
Post Number: 921
Registered: 12-2001
| | Posted on Saturday, October 05, 2013 - 12:06 pm: |
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Charliebrown, how much loss do you have on that 500 feet of feedline, have you ever checked it. I'm going through about 275 feet total to the radio in my house, RG-8. I've got about 40% loss on that amount. My tower is out at my shop and when I use a radio there I'm just going through 75 feet of coax to the antenna. But you know people on the other end can't tell much difference if I'm out there or in the house using the same output. |
Charliebrown
Intermediate Member
Username: Charliebrown
Post Number: 289
Registered: 10-2002
| | Posted on Sunday, October 06, 2013 - 6:48 pm: |
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From what I have found on the Net, the loss per 100 foot is about 0.4. I have about six splice's in the 500 foot. I do not know how to figure the loss for each splice. I do not have anything like some of the ham's or tech's do to measure the loss. I guess I could use a watt meter at the end of the coax to see how much power loss there is but, all I know is that I get out well. I would like to just get out a little better and have a little bit better receive without spending any more money. As close as I can figure I am most likely getting about half or 60 percent of my power to the antenna. |
Tech237
Moderator
Username: Tech237
Post Number: 1703
Registered: 4-2004
| | Posted on Monday, October 07, 2013 - 10:39 am: |
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Any loos is too much. Any operator should always try to reduce losses as much as possible. Remember losses work both ways - transmit and receive. Ever wondered how many stations you miss hearing because of feedline loss??
Scrapiron - I bet you miss stations on the longer run that you could hear on the shorter one. Not close by stations, who could hear you on the old 6" wet string, but those further afield.
Again, maybe not as much a problem around 30Mhz, but really starts to be one at higher frequencies.Remember loss per 100ft increases with frequency.
Rule of thumb - always use the best coax you can afford. Keep your losses as low as possible, and try to avoid joins in the feedline.
Tech237
N7AUS
God made me an athiest, who are you to question his wisdom?
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Scrapiron63
Advanced Member
Username: Scrapiron63
Post Number: 922
Registered: 12-2001
| | Posted on Tuesday, October 08, 2013 - 9:02 am: |
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Charliebrown my method to check the loss probably wasn't too scientific but I used a wattmeter and dummy load. I first hooked them to the radio and set the output of my radio to 20 watts. I then hooked the wattmeter and dummy load to the end of the feed line. 20 watts in the house got 12 watts to the antenna. |
