Using 75 ohm coax with50 ohm bnc connector !? - Page 1
Forum section: electronics / rf-microwave

author: Dom13c
author_rank: Regular Contributor
timestamp: January 19, 2020, 05:03:42 pm
content: As I've previously posted, I lack 50 ohm rg-58 coax cable and people down here selling them don't even know what coax cable impedance is and why it matters !So all what I can get is the 75 ohm type. ( know this as all the coax sold here are mostly used for satellite TV receiving equipments. ) but the bnc connectors available here are all 50 ohm type.Now my question is, is it possible to use a balun transformer between the bnc connector and the coax for 50 to 75 ohm impedance matching ? ( preety stupid question, I know)I'm gonna use the connectors form taking output form a frequency generator to receive and transmit with qrp rigs( Pout 10watt max)

author: SeanB
author_rank: Super Contributor
timestamp: January 19, 2020, 06:00:44 pm
content: At that level it makes little difference, your SWR will be marginally worse, your input matching a little worse, but it will work for all but the most marginal signals. Your transmitter will barely notice the difference with only 10W, though you might find your coax cable is a little lacking in the shielding performance, so i would recommend putting a 10 turn ( over a roughly 10cm diameter former, typical tin can size) coax choke at the transmitter and antenna end to tame it down a little, so the shield does not radiate so strongly.

author: tkamiya
author_rank: Super Contributor
timestamp: January 20, 2020, 05:36:46 am
content: One of the problems you will face is the size differences of coax. RG59 and RG6 is a lot bigger than RG58. They are more like size of RG8X. You *could* use balun (custom made) or least loss converter using resisters, but I would personally just do it, and not be concerned about the impedance difference.If you are going to use it in ham band, VSWR of 1.5 won't make a least bit of difference. Over the years, impedance of BNC has become very vague. If you'd go to Amazon or eBay, connectors of both types are sold almost interchangeably with random cables.Where are you located and are you looking for crimp type or clamp type? For what coax do you need them for? I might have a few I can send to you.

author: helius
author_rank: Super Contributor
timestamp: January 20, 2020, 07:32:08 am
content: Traditionally 75-ohm coax was used for analog video (CATV, VHF/UHF OTA), and the connector designed for that was the type F.The Type F is not very reliable under industrial conditions, so many professional devices used BNC. However, the vast majority of BNC connectors are 50-ohm characteristic impedance, so they will cause reflections on a 75-ohm coax cable. This mismatch was not considered significant due to the signal quality problems inherent in analog video, so that was the state of things for a long time.When digital video was introduced, 75-ohm coax was chosen for it (SDI), but now the 50-ohm BNC connector was found to be causing errors, so "True 75-ohm" BNC connectors came on the market. These have reduced dielectric inside the female connector, but will still mate with the 50-ohm type, so you need to check that you are using the correct type on both sides.

author: ardf
author_rank: Contributor
timestamp: January 20, 2020, 08:36:49 am
content: There are BNC to F connectors there like:https://www.amphenolrf.com/aph-bncp-fj.htmlfor 1$ or less (Chinese made) that are useful for your purpose.The losses on RG-8 are substantially less than in RG-58, so for higher frequencies, it's even advisable to use RG-8 despite the slight impedance mismatch.

author: bd139
author_rank: Super Contributor
timestamp: January 20, 2020, 08:45:14 am
content: I wouldn’t bother with RG8. I actually measured loss on a 20m run of that versus some decent RG58 and it was in the order of 1dB at 7Mhz. Hardly worth worrying about.Was this stuffhttps://rover.ebay.com/rover/0/0/0?mpre=https%3A%2F%2Fwww.ebay.co.uk%2Fulk%2Fitm%2F273808075263But this stuff is available in large quantities everywhere via eBay locally and from China.My portable antenna has 7m of RG174 (from sotabeams). No problems making contact. What’s at the end is more important!Also don’t use F connectors. That’s how you lose your PA transistors!

author: tggzzz
author_rank: Super Contributor
timestamp: January 20, 2020, 09:11:58 am
content: Be aware that 50 and 75 ohm BNC connectors have subtly different internal dimensions and may damage each other.Apart from that, you give insufficient details of your application and design for us to form an opinion. A frequency generator with a 10W output doesn't feel right.

author: bd139
author_rank: Super Contributor
timestamp: January 20, 2020, 09:22:44 am
content: It’s a transmitter

author: radiolistener
author_rank: Super Contributor
timestamp: January 20, 2020, 09:41:54 am
content: it depends on the use case. If you're using it for short wave radio where frequencies is smaller than 30 MHz, then bnc connector with a little different impedance doesn't play significant role, because bnc connector size is to small in comparison with wave length.But if you're want to use it for satellite communications at GHz range, it will significantly affect your connection, because bnc connector size is comparable to wave length.

author: tggzzz
author_rank: Super Contributor
timestamp: January 20, 2020, 09:46:48 am
content: Quote from: bd139 on January 20, 2020, 09:22:44 amYeah, but good luck getting 60GHz through coax and a connector, or finding which HP frequency generators have 10W output

author: vk6zgo
author_rank: Super Contributor
timestamp: January 20, 2020, 12:49:58 pm
content: Quote from: helius on January 20, 2020, 07:32:08 amYou got in before me!There must still be a lot of 50-ohm RG59 compatible coax connectors around, because we used them in their thousands, & even more of those for the thicker twin shielded 75 -ohm coax (can't remember the part number), which was used throughout the TV Studio I worked in.The "AR" magazine in Oz (unfortunately, I don't remember the issue), had an article on using 50-ohm BNC crimp connectorsdesigned for RG58with RG6.It is a bit "fiddly" but it can be done----I did one, just to see if it worked, but have never used it.Of course, you can buy BNC crimp connectors specifically intended for RG6, but the RG58 ones are much more common.

author: macboy
author_rank: Super Contributor
timestamp: January 21, 2020, 01:10:51 pm
content: Quote from: tggzzz on January 20, 2020, 09:11:58 amUntrue! You might be thinking of N type connectors where the dimensions do differ and damage can occur. For BNC, all metal part dimensions are identical, and the only difference is the presence (50 Ohm) or lack (75 Ohm) of dielectric between the pin or receptacle and the shield within the connector body. Someone almost always chimes in with this false warning whenever 75 vs 50 Ohm BNC is brought up, but there is no need for concern, from a mechanical standpoint.IMHO the OP should be far more concerned about the mismatch between the 75 Ohm cable and the signal generator which likely has a 50 Ohm output.

author: radiolistener
author_rank: Super Contributor
timestamp: January 21, 2020, 03:10:28 pm
content: Quote from: macboy on January 21, 2020, 01:10:51 pmThere is no problem with mismatch between 50 Ohm sig gen and 75 coax, because sig gen is a source. The problem happens with impedance matching on the load side of coax line. For example, when you use transmitter (source) and antenna (load), and antenna is not matched with coax line, your transmitter will get reflected wave from antenna.

author: fourfathom
author_rank: Super Contributor
timestamp: January 22, 2020, 07:55:49 am
content: Quote from: radiolistener on January 21, 2020, 03:10:28 pmOf course there can be a problem with mismatch. With a 50-ohm source, the 75-ohm cable will act as an impedance transformer. The nature of the transformation (resistive and reactive) will depend on the length of the cable and of course the frequency of operation. The load (antenna) may or may not match this transformed impedance and if it doesn't there will be reflections. In practice and especially at lower frequencies these effects may not be serious.

author: radiolistener
author_rank: Super Contributor
timestamp: January 22, 2020, 11:32:03 am
content: Quote from: fourfathom on January 22, 2020, 07:55:49 amNo. Since 75-ohm cable is loaded with 75-ohm load there is no reflected wave in the cable and the cable doesn't works as impedance transformer even if the source impedance is 50-ohm.Reflected wave in the cable depends on impedance match between cable impedance and load impedance. It doesn't depends on source impedance.When the cable is matched with the load on the cable output, cable input impedance will be always equals to cable impedance and there is no impedance transformation. And it doesn't depends on signal source impedance.The same as battery internal impedance doesn't affects load resistor impedance which is connected to the battery. When you connect 15 Ohm resistor to the battery, it will be always 15 Ohm and it doesn't matter what is internal impedance of the battery

author: fourfathom
author_rank: Super Contributor
timestamp: January 22, 2020, 04:52:01 pm
content: Quote from: radiolistener on January 22, 2020, 11:32:03 amYou are correct that there is no transformer action if the load matches the Z of the transmission line (I didn't notice if that was the case here). In the 75-ohm line and load situation there will be no reflected power, but the 50 Ohm Zo generator (transmitter) will see a 75 Ohm load which is still an impedance mismatch. The transmitter may or may not care about this amount of mismatch. (SWR = 1.5:1)

author: radiolistener
author_rank: Super Contributor
timestamp: January 22, 2020, 06:03:01 pm
content: Quote from: fourfathom on January 22, 2020, 04:52:01 pmYes, generator may not like to work on load impedance within some value range (usually it doesn't like too low load impedance). But this is a problem inside generator. Not coax line

author: Dom13c
author_rank: Regular Contributor
timestamp: January 24, 2020, 06:36:50 pm
content: Quote from: tkamiya on January 20, 2020, 05:36:46 amtThans for having thoughts to help me out. I'm from India and sending even a nail from US would cost 10 times the price of its own. So, best would be to suggest an alternative . W hat I have is 75 ohm coax and 50 ohm bnc's. Using them tgether is what has turned into a PITA for me

author: Dom13c
author_rank: Regular Contributor
timestamp: January 24, 2020, 06:52:51 pm
content: Quote from: tggzzz on January 20, 2020, 09:11:58 amI messes up with the typing. Missed a comma.I talked about 10 watt output qrp rigs and taking output from frequency generators.What I meant in a nutshell is, I've got 50 ohm type connectors. And 75 ohm cables. And I'm 100 % sure that those connectors are ( both jack and plug) 50 ohm variety as they both have a thick ring of dielectric material around the core.What I can't figure out is how to connect the 75 ohm coax with 50 ohm connector avoiding any kind of mismatch.Actually don't want to change the bnc's cause 75 ohm bnc's are rare as hen's teeth here. And I don't really know where to look for for 50 ohm coax. Ordering online is a risk as if the particular stuff I order turns out to be wrong variety stuff it would be a total waste and I already have a decent supply of 75 ohm type coax for dirt cheap ( 10cent a yard) from a cctv installation guy.

author: Dom13c
author_rank: Regular Contributor
timestamp: January 24, 2020, 06:56:58 pm
content: Quote from: radiolistener on January 20, 2020, 09:41:54 amI do HF only as I lack any VHF rig or handheld and I purely make my own rigs (too ugly lookingbut works )So there is a very little chance of me doing uhf.Should it be fine then to directly crimp the 75 coax with the 50 ohm connector?

author: Dom13c
author_rank: Regular Contributor
timestamp: January 24, 2020, 07:06:52 pm
content: Quote from: fourfathom on January 22, 2020, 04:52:01 pmWhat about reception ? Would it get hamperde too due to signal loss for mismatching?

author: tkamiya
author_rank: Super Contributor
timestamp: January 24, 2020, 07:26:30 pm
content: tThans for having thoughts to help me out. I'm from India and sending even a nail from US would cost 10 times the price of its own. So, best would be to suggest an alternative . W hat I have is 75 ohm coax and 50 ohm bnc's. Using them tgether is what has turned into a PITA for meThat's too bad.... I have hundreds of them in a box. They are all crimp type though. How about giving up on direct connection? If it's 75 ohm coax, F type connector will be a right fit. Then you can buy a bunch of F to BNC or whatever converter. I've done it few times and it works very well.

author: tkamiya
author_rank: Super Contributor
timestamp: January 24, 2020, 07:51:39 pm
content: Does anyone here know an least expensive way to send from USA to India? I'd say 10 pounds of connectors..... We do enough trade with them that there got to be a cheap way to do this....

author: ardf
author_rank: Contributor
timestamp: January 25, 2020, 09:29:18 am
content: Quote from: Dom13c on January 24, 2020, 06:56:58 pmAs you've mentioned you have an access to the cheap 75 Ohm coax. Try to avoid of using the 75 Ohm coax with sparse braid as it's performance could be discouraging. The foil shielding that is Ok on VHF sometimes do a poor job on HF.

author: Dom13c
author_rank: Regular Contributor
timestamp: January 26, 2020, 07:51:53 am
content: Quote from: ardf on January 25, 2020, 09:29:18 amGiven that I use F connectors, would it be fine to tweak the output/ input LPF, BPF to match the 75 ohm impedance of the F connectors?

author: ardf
author_rank: Contributor
timestamp: January 26, 2020, 03:49:34 pm
content: Quote from: Dom13c on January 26, 2020, 07:51:53 amThat's exactly what I do with ARDF transmitters, when the antenna impedance is not 50 Ohms. This may affect the transmitter spectrum purity, but I have no spectrum analyser to check. To this time I've got no complaints about possible spurious emissions from other hams, though.

author: fourfathom
author_rank: Super Contributor
timestamp: January 26, 2020, 04:02:24 pm
content: At HF frequencies you could use alligator clips for your connectors without much ill effect. The long 75-ohm cable can be a factor, depending on source and load impedance but most HF transmitters will barely notice the difference.Obviously you want secure connections, so the cable and connector should physically fit.

author: tkamiya
author_rank: Super Contributor
timestamp: January 26, 2020, 05:42:37 pm
content: No. It's not necessary. Your radio won't know the difference. Filters are difficult to tune without proper equipment and knowledge. You are more likely to de-tune it and get lost than improve it. If you desire, you can use antenna tuner. (easy and fun project) and use that.What kind of radio do you have? You may already have a built-in tuner.By the way, I used to use terminal strip to connect coax. My radios were just as happy. On HF, not much matters.

author: Dom13c
author_rank: Regular Contributor
timestamp: January 26, 2020, 06:52:16 pm
content: Quote from: fourfathom on January 26, 2020, 04:02:24 pmThanks,.Just for that 'non critical ' side, I prefer hf over vhf for Homebrew equipments. The connectors I have are non crimp type. So the cable size won't be much of a problem for me actually

author: Dom13c
author_rank: Regular Contributor
timestamp: January 26, 2020, 07:04:01 pm
content: Quote from: tkamiya on January 26, 2020, 05:42:37 pmwhat I had were some scratch bulit qrp transceivers. So, up untill now I was happily using an end fed antenna combined with a T match ( polyvaricon capacitors and air core inductor). But now that I've got myself a uBITx, I wanted to switch to a dipole. That's what made me hustle to buy the coax and connectors and made the whole thing a mess. I was actually worried about the impedance mismatch as at qrp level losing even a couple of hundreda of milliwattsfor mismatch means losing a contact.

author: tkamiya
author_rank: Super Contributor
timestamp: January 26, 2020, 07:35:45 pm
content: Even at QRP level, loss is negligible. I really would not worry about it. You CAN use end fed wire with good ground if you have an antenna match. (that's what I have here) Loss is measured by DB anyway. So its effect is the same either you are QRP or QRO.F connectors were designed for up to UHF frequencies. Loss at HF is almost none. Obviously, it is not appropriate for high power but you don't have to worry about that.I am a ham as well. I used to worry about loss caused by PL259/SO239 and considered going to N connector. By the way.... connector is very short and small relative to your cable. Impedance is dominated by cable rather than connector. Even then, it makes no difference. RG6 has better (smaller) loss than RG58 even with mismatch.

author: tkamiya
author_rank: Super Contributor
timestamp: January 26, 2020, 11:46:27 pm
content: Now you guys got me curious. I want to find out for certain.I've just ordered F to BNC adapter. It should be here in few days. I will put those and few feet of RG6 to the test on my spectrum analyzer and tracking generator. Then compare that against BNC to BNC 50 ohm. I can scan it from near DC to 100MHz then extend the top end to 1.8GHz. Let's see what it will show. I will be back in few days with results.I've ordered no brand name adapter from Amazon so it should be a representative of basically any-junk. They will not be the top quality stuff.

author: Dom13c
author_rank: Regular Contributor
timestamp: January 28, 2020, 01:09:26 pm
content: Quote from: tkamiya on January 26, 2020, 11:46:27 pmHell yeah! That would be awesome!.If possible try to find out the loss level when 75 ohm type coax is directly connected to 50 ohm type BNC connector without any matching!

author: tkamiya
author_rank: Super Contributor
timestamp: January 28, 2020, 03:21:04 pm
content: I can already tell you that will be immeasurably small in HF range via the method I mentioned. I would have to do something a bit different. What is the highest frequency you'd be interested in? How long of 75ohm coax would you be using?

author: tkamiya
author_rank: Super Contributor
timestamp: January 28, 2020, 10:50:42 pm
content: OK, I have some what of a result but this is not done in ideal condition. The F to BNC adapter I received is in bad quality. And the cable I used was RG59. Single layer braid and no foil. But I went ahead and done some measurement anyway.All of my test equipment uses N connector as input and output. Keep this in mind. I had to change that to F connector somehow. N to F does not exist from where I buy my stuff, so I had to chain a bunch of connectors. One on side, there is N to BNC, BNC to F connector. So it involves 2 adapters. On the other, N to N (gender change), N to BNC, BNC to F. So it goes through 3 connectors. That's whole bunch of adapters.Using tracking generator and a spectrum analyzer, from 0.5 to 30MHz, loss of 1 meter F to F cable was not detectable. Only over 1GHz, I saw some decline but that of far better than what I normally see from RG58 type.Then, I used RF generator and power meter. I get far more accuracy and resolution this way. Connecting them directly, I read -0.03dbm. Attaching the same cable assembly I described above, I read -0.58dbm. So the whole assembly + cable + mismatch loss of 50/75 ohm TWICE resulted in 0.55db.In term of power, that means if you put 100 watts into this, you get 89 watts out. One S meter difference is 6db by the way.Let me see if I can get proper connectors and retest. There are too many variables in my test setup. THIS, however, represents perhaps the worse case scenario. The loss reading, however very low, look too high to me. I was expecting half that.I will report back later.

author: Dom13c
author_rank: Regular Contributor
timestamp: January 30, 2020, 12:15:10 pm
content: Quote from: tkamiya on January 28, 2020, 03:21:04 pmHighest frequency would be 54 MHz ( for transmission and reception both)/144 MHz( for reception only till I get myself a nice VHF transceiver)The cable run would be around 50 feet from my shack to the antenna.

author: Dom13c
author_rank: Regular Contributor
timestamp: January 30, 2020, 12:28:32 pm
content: Quote from: tkamiya on January 28, 2020, 10:50:42 pmThank you for investing your precious time to look into the topic.I'll be looking forward for the results.

author: Martin.M
author_rank: Frequent Contributor
timestamp: February 03, 2020, 12:02:30 pm
content: Quote from: macboy on January 21, 2020, 01:10:51 pmsorry, the diameter of the center pin is various

author: tkamiya
author_rank: Super Contributor
timestamp: February 04, 2020, 04:37:04 am
content: Martin,BNC connectors for 50 ohm and 75 ohm are supposed to be interchangeable. N types aren't interchangeable.In case of BNC, the outer dimension of the pin may be different, but the tapered portion should be the same. Otherwise, they won't mate without some damage to either or both side.Dom13C,All parts are here. This time, I bought some good adapters from Digi-key. Unfortunately, I've been ill and unable to spend time in my lab. I will try to get to it soon. I am very curious on this one myself. In old days, I did some crazy things like using AC plug for connecting coax. Now, I have all the equipment available to me, I will take some measurement and really find out if I was crazy or it just doesn't matter much.

author: Dom13c
author_rank: Regular Contributor
timestamp: February 06, 2020, 07:43:20 am
content: [quote author=tkamiya link=topic=226278.msg2902030#msg2902030 date=1580791024Dom13C,All parts are here. This time, I bought some good adapters from Digi-key. Unfortunately, I've been ill and unable to spend time in my lab. I will try to get to it soon. I am very curious on this one myself. In old days, I did some crazy things like using AC plug for connecting coax. Now, I have all the equipment available to me, I will take some measurement and really find out if I was crazy or it just doesn't matter much.Thank you again!I'm really in no hurry. So, please don't waste any valuable time. Wait till you get free time. I'll be glad to know the numerals from you after being done doing the experimentsJust out of of curiosity, what do you do for a living and how long it has taken you to acquire all those equipments ?I'm still a student and find myself helpless while I can't have an insight of what I'm building without any equipment . so your story of struggle might inspire me to be patient.

author: tkamiya
author_rank: Super Contributor
timestamp: February 06, 2020, 04:34:30 pm
content: I am a software developer / programmer during day time.During my early adulthood, I was poor. My first computer(s) were picked up from flea market (kind of place where junk were sold) and I fixed them. Out of 8, I got 6 working. All for $25! I still pick up electronics thrown away. If anything, they are good for parts.My test gears are not that much different. I have a room full of HP equipment. Most of them were purchased broken / with problems / in sad shape for very little money. Sometimes I get 2 or 3 and make one working ones. Sometimes I spend days repairing one. The fact I have a calibration / repair lab with friendly people helps.I only wish if you were closer.... (to US) I can give you trunk full of stuff.Right now, I'm fighting an issue that makes me very difficult to concentrate. It comes and goes. When it clears, I will do the testing and we can discuss it.By the way.... I am a ham also. I started when I was 12 or 13. I used anything for antennas. When I couldn't afford copper wire, I used vinyl coated steel wire. I pieced together shorter junk coax to longer ones. This is with 10 watt transceiver. Anything works. I didn't notice any difference. HF is very forgiving.My first beam antenna was made out of 300 ohm TV feeder, bamboo sticks, and a piece of wood. It worked GREAT! Look up ZL special. Cheap, easy, and very good working antenna. (also it matches 75 ohm)

author: Dom13c
author_rank: Regular Contributor
timestamp: February 06, 2020, 05:17:36 pm
content: Quote from: tkamiya on February 06, 2020, 04:34:30 pmThanks for giving time and reference of ZL special antenna. That helpful people like you exist makes this earth a better place.All these pretty much explains me ,too. And yes living down here in India sucks like hell. Cut off from everything good.And definitely, have your problems solved, then only do the testings

author: tkamiya
author_rank: Super Contributor
timestamp: February 07, 2020, 12:32:08 am
content: I have a result.I used a little different method to measure this. I used my spectrum analyzer and tracking generator and scanned 1 to 30MHz, which represents the entire HF band. Looking at pictures below, left most vertical line represents 1MHz. Right most line represents 30MHz. On this picture, the line is showing what the cable assembly will act like for every frequency on the HF band.Looking at horizontal line, each line represents 1dB loss, as you go down. Very top line is not used. (frame of the rectangle) The second line is the reference line = 0 dB. Rather, the trace is 0 dB the way I set this up. Waviness of the line is caused by my equipment.The cable you asked for is constructed as follows.Cable itself is 50cm piece of RG6. I used short piece of cable because your interest is in connectors, not the cable. On both end, there are F connectors. On both end, there are BNC to F connectors. So this test includes the effect of both F and BNC connector used together on both ends.Reference cable is 50cm piece of RG58. On both end, there are BNC connectors. The purpose of this cable is to calibrate my setup - it represents PERFECT connection.On spectrum analyzer and trackGen, there are N to BNC adapter connectors. As you can see, this way, I can attach connectors to the spectrum analyzer as they share BNC as common port type. By the way, 50 ohm and 75 ohm BNC are inter-mix-able. Same is NOT true for other connectors.Step 1: Connect reference cable to specAn and trackGen.Step 2: Adjust output so that second line from top represents ZERO loss. Please note, the line is not completely straight. This is NOT an effect of the test cable. This is error inherent on my specAn and trackGen. We are talking about 1 dB per scale here. Nothing is perfect.Step 3: Take a picture. (time stamp on top left corner 17:40:03) This picture represents PERFECT connection. (although it is not)Step 4: Remove reference cable and replace it with RG6 cable as described above.Step 5: Take a picture. (time stamp on top left corner 17:39:38)1 dB loss represents 100 watts becoming 80 watts. 1 watt becoming 0.8 watt. 20% loss.0.5 dB loss is 11%0.1 dB loss is 2.3%ONE S-meter change is 6 dBNow I ask you this.... Do you see any difference? I don't.And.... remember, we have both F and BNC connectors on both ends.I can basically conclude, in HF band, effect of using F connector and BNC connector is so negligible.By the way.... nothing is lossless. So the loss is there. But it's smaller than 0.1dB. Thickness of the trace is about 0.1dB! I will gladly use this for my station.An only thing I'd mention is, quality of F to BNC connectors varied greatly. For the first test, I used a no-name cheapo. It was awful. It was lose. I could not get a stable reading. For the second test, I bought a quality connector from respected source. Even then, it was less than 5 dollars a piece. When making purchase, don't go for the cheapest. Test it out first....I did not test RG6 connected directly to BNC. I know it is not going to be measurable.

author: tkamiya
author_rank: Super Contributor
timestamp: February 07, 2020, 02:07:45 am
content: For fun, I did the same thing I did the first time. Used signal generator and power meter.Set the signal generator to 30MHz. Using "reference cable", set the power meter to read 0 dBm.Change the cable to RG6 and F connector assembly. Now I read -0.12dBm.This includes the loss in the cable (RG6 50cm) itself.0.12dB loss is same thing as slightly less than 3% loss. Again, this includes cable itself and TWO of F connectors. Hardly anything to worry about in HF.

author: Dom13c
author_rank: Regular Contributor
timestamp: February 08, 2020, 04:40:19 pm
content: These tests says everything. Now I can live in peace and do qrp without worrying about excessive signal attenuation.Heartily thank you for investing your time to do the tests. Especially the very last line ' It's not going to be measurable ' was the most satisfying.Stay well.73Dom13C

author: radiolistener
author_rank: Super Contributor
timestamp: February 10, 2020, 10:37:21 pm
content: Quote from: tkamiya on February 07, 2020, 02:07:45 amyes, it means that if you put 100 W through cable, 3 W will be used for heating the cable and connectors.But note, this is actual if you're using ideal dummy load with VSWR=1.000 on the second side of the cable. If you're working on the real antenna which has VSWR > 1, the loss will be much higher and will depends on VSWR, because 3% of the power loss will be applied to each reflected wave in the cable...

author: tkamiya
author_rank: Super Contributor
timestamp: February 10, 2020, 10:58:08 pm
content: I really didn't want to get into that, since he is a newly licensed ham.He is actually only interested in impedance mismatch at the connector, (ie. using 75 ohm BNC verses 50 ohm BNC) Even then, unless the mismatch is horrible, actual impact on HF will be negligible.

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: February 16, 2020, 09:09:28 pm
content: I did not see what type of antenna you are using, maybe I did not read enough of the posts.An advantage of 75 ohm antenna lead is thatdipoles are 73 ohms. So the feed line is better matched to the antenna.Also usually 75 ohm has less loss and would be better for QRP.You will probably get a better all around result with 75 ohm cable if you are using a dipole.If you are using a modern HF rig you may want to use an impedance transformer at the radio.For a 50 ohm output radio you would use a torroid and 4 wraps of wire on the radio side and 5 on the 75 ohm antenna side. You would use this at the radio. Using a dipole and the 75 ohm coax and the UNUN you will have a better balanced antenna system than probably 99% of hams.But it is probably not necessary to use the UNUN (unbalanced to unbalanced transformer).If you are using an old radio or one with a tube output, you do not need any sort of transformer at all.I think that most of this was covered by others but I could not resist putting in my 2 cents.The fellow I know who has the most professional experience with RF engineering has 75 ohm cable to all of his antennas.Frankly I do not see any real difference in using BNC connectors, although there is a more or less theoretical one.The connector we hams use most often is the "UHF" PL type connector. That connector is not 50 ohms at any frequency that I know of. (maybe there is one freq that it is 50 ohms.....). It is more like 32 ohms at 10 meters. Go think....That connector predates WW II when UHF was maybe anything above a few megahertz.Whatever connector is on your radio is the connector I would use.Wally KC9INK

author: Yansi
author_rank: Super Contributor
timestamp: February 16, 2020, 09:14:36 pm
content: Quote from: tkamiya on February 07, 2020, 12:32:08 amWith 50cm cable at 30MHz you won't see a difference. You could use even a piece of phone line with similar result.Try 3GHz and report back.

author: radiolistener
author_rank: Super Contributor
timestamp: February 17, 2020, 06:13:46 am
content: Quote from: Yansi on February 16, 2020, 09:14:36 pmHere is measurement of two pieces of wires with 30 cm length in 10 kHz - 30 MHz frequency range:The difference is very visible. But how much it affects your needs it depends on your requirements.As we can see, 3 dB bandwidth of 30 cm wires is about 25 MHz.