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Old 06-29-2017, 3:13 PM
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Default Question about coax

I finally got around to getting an ICOM 7200 as my first HF rig. I happen to have some 70 Ohm coax. Just out of curiosity, can I use this coax with my ICOM 7200?

From what little I think I understand the only impact might be on the SWR. Does this mean anything for receiving or only transmitting?
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Old 06-29-2017, 3:48 PM
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High SWR can destroy the finals , you can run 70 or 75 ohm coax for HF however from the start your SWR will be around 1.5:1 so anything else wrong as far as Antenna and such will bring it up , for safest results run it through a tuner ,and do not try and use it on a frequency higher than around air band 100 megahertz .

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Old 06-29-2017, 4:06 PM
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ICOM 7200 IS designed for 50 ohm cable and antennas.
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Old 06-29-2017, 4:41 PM
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Quote:
Originally Posted by tdenfuny View Post
I finally got around to getting an ICOM 7200 as my first HF rig. I happen to have some 70 Ohm coax. Just out of curiosity, can I use this coax with my ICOM 7200?

From what little I think I understand the only impact might be on the SWR. Does this mean anything for receiving or only transmitting?
Plenty of amateurs have used old 75Ω cable company hard line. As stated it will have a higher SWR, but the difference between 50Ω and 75Ω alone isn't a show stopper.
SWR isn't the only thing that makes a good antenna, so don't get too hung up on a 1.5:1 vs. 1.0:1 SWR.

Not ideal, but your radio will not explode into a fireball/mushroom cloud if the SWR is a bit high. Usually radios will handle a certain amount of reflected power, to a point, after that point they will usually start turning down the PA output. Long term use can/will cause damage, so you do need to keep the SWR in acceptable limits.
Ideally, you do want 50Ω coax for antennas that will take or are designed for 50Ω.
The antenna tuner option is good advice.
You can also make a 50Ω to 75Ω matching setup.

As for using it to receive, it won't hurt anything. Where the higher SWR becomes a risk is ONLY when you are transmitting.
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Old 06-29-2017, 5:23 PM
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Quote:
Originally Posted by mmckenna View Post
...
Ideally, you do want 50Ω coax for antennas that will take or are designed for 50Ω.
The antenna tuner option is good advice.
You can also make a 50Ω to 75Ω matching setup.
Thanks for all the replies.

So, just to make sure that I understand: will an antenna tuner actually protect the radio in this situation?
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Old 06-29-2017, 5:39 PM
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Quote:
Originally Posted by tdenfuny View Post
Thanks for all the replies.

So, just to make sure that I understand: will an antenna tuner actually protect the radio in this situation?
Maybe, if it's set up correctly.

You'll need an SWR meter between your radio and the antenna tuner to tell you that. Your rig may have one built in.
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Old 06-29-2017, 5:40 PM
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An antenna tuner is nice to have because its rare to find an antenna that covers every band with a good enough match to use without a tuner.

One thing about using 75 ohm coax is, if you use a 1/2 wave dipole at 1/2 wavelength or multiples of 1/2 wavelength above ground, its impedance should be close to 70 ohms. In that case your radio should be able to feed that with no tuner, as there will be little to no reflected power and the radio should be ok with that.
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Originally Posted by tdenfuny View Post
Thanks for all the replies.

So, just to make sure that I understand: will an antenna tuner actually protect the radio in this situation?
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Old 06-29-2017, 7:59 PM
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the impedance of HF antennas is rarely near 50 ohms anyway that's why everybody uses tuners

So splitting hairs over a few ohms is a moot point
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Old 06-30-2017, 6:43 PM
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But, good practice IS to check the SWR first on an unknown antenna and it's cable.
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Old 07-01-2017, 12:51 AM
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Tden, personally I would not worry at all about using the ~70 Ohm cable with your Icom. Even if there is a mismatch between the transmitter and the coax of a 1:1.5 SWR, that represent a reflected power of <5%. The amount of reflected power seen back at the transmitter's PA will be decreased (burn'd up) by the coax inherent Ohmic losses- and thus pose little strain on the amplifier. Modern transmitters have fail safe thermal run-away circuits to prevent their amplifier's destruction- the most you would see with a badly coupled line would be a decrease in power, as evidenced on your power output meter. At a 1:1.5 SWR you won't see a thing----- A 100 Watt transceiver can well handle 72 ohm coax without the need for any tuner.
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Something to keep in mind- 'Coax cable' is just another term for a RF wave guide.... that's where the "RG" in RG-213 (etc.) comes from. While it can be rated at a certain Ohmic impedence- it does not always exhibit that value. For instance: when a line length is in even multiples of half wave lengths, the input resistance will be equal to the load resistance- regardless of the lines impedence. Thus if you have a half wave long (but don't forget the velocity factor in calculating this length) line matching a resistive 50 Ohm antenna, it doesn't matter if the coax is 72 Ohms, it will be a "prefect match."
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That was but one of the many quirks of coax'es -- feedlines actually. I am getting a little too technical, No?... sorry
When I get like this its time for an Anecdote.
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In my lab we have ten or twelve (I forget how many) 1000 foot spools of Belden 8213 coax. Their origins predate my tenure- but that's over 2 miles of cable !- what its original purpose was to be, I haven't a clue. Anyway, its 72 Ohm, beautiful foam dielectric 1.5db loss/100 feet @100 Mhz's line.... and we have plenty of it. And because we have so much of it, my coax budget is $0- and will probably remain zero for a long time to come (ie: no general purpose 50 Ohm stuff in our future )
This cable is used for every routine RF run imaginable- and with very few exceptions, no one questions whether its to match a 50 or a 72 (or there abouts) loads. It works fine, and this spans the gamut of frequencies- from DC to ~3 Ghz, and high powers too.
Its been my experience that few antennas present a perfect "50" or "72" Ohm load. So if, say, your SWR is 1:1.5 with a 72 Ohm cable- 'tweak' the antenna with a little +/- inductance/capacitance- and "Bingo" ---- you can achieve a 1 to 1.
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Go ahead and use your 72 Ohm stuff Tden....
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.
......................................CF
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Old 07-01-2017, 4:00 AM
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Coax here on my 40/10 meter dipole, 6 meter dipole, and discone all RG-6 (75 ohm)... all work great.
The stuff is readily available.. you can even find it at most any hardware store, easy to work with... installing "F" connectors is a snap, no soldering, and it's cheap.
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Old 07-01-2017, 7:23 AM
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Thanks again for the replies. I did try out the 75Ohm coax and the ICOM 7200 built in SWR meter gave me a 1:1. I ordered some 50 Ohm coax also.
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Old 07-01-2017, 7:57 AM
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I have a home made 1/2 wave dipole made out of 3/8" aluminum tube cut to 156 mhz and the 75 ohm RG 6 didn't function nearly as well as the RG 58 50 ohm. The RG 6 had good RSSI but was only for stations within 30 miles, the RG 58 had great RSSI near area and in the 600's up to 80 miles out. The RG 6 simply would not receive distant signals.
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Old 07-01-2017, 12:40 PM
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Hmmm.... Something seems a bit off ChrisABQ. RG6 coax has a lower loss than RG58 (or RG59)-- that's a reason its popular for the line between the LNC/Dish and the the 900 Mhz tuner in home and commerical satellite setup up's.
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RG6, for instance, at 156Mhz-- a 100 foot run's loss is 3.4 dbs, while RG58's loss will be 5.7 dbs (46 v.s 27 % energy transfer efficiency.)
.............And no, I don't carry these numbers in my head- but I do have this Book Mark on my tablet.........
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http://www.timesmicrowave.com/calculator
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I wouldn't disparage the the RG6,-- though all RG6's may not be equal- as are RG58's, 59's-- et al. Anyone remember the Radio Shack quality coax cables that they sold during the CB crazy days?... a bit before me, but I had a 200 foot run of RS RG58 given me years ago. I used it to feed my 160 meter (highly load-coil'd) vertical for years-- you could see a lot of 'daylight' between the copper shield'd braid- but at 1.8 Mhz it didn't matter much. I wouldn't say the same thing if it were used on V/UHF.
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It may be the RG6 cable's fault, but then I'd look at other things first before condemning it-- connectors, age, condition---?? Certainly on paper it should preform better than '58, though <3 db difference will be barely, if at all, noticeable.
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.................................CF
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