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Feedline Connector Loss

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by K2POP, May 21, 2011.

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  1. K2POP

    K2POP Ham Member QRZ Page

    [FONT=&quot]For my 70cm antenna I am using LMR-400 coax and N connectors. I would like to use two 90° N Elbows in the line. Usually, large radius turns are best in transmission lines. How much line-loss will these elbows create? How much loss do straight connectors create?

    Richard, K2POP
  2. W0LPQ

    W0LPQ Ham Member QRZ Page

    You probably would not be able to measure the loss.
  3. KA4DPO

    KA4DPO Ham Member QRZ Page

    Type N connectors are extremely low loss and even the 90 deg connector would have less than 1/10th of one db loss. Not even worth worrying about.
  4. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    Properly installed high-quality type N's have no more loss than the amount of coax they replace, which is about 1-1/2" worth. Elbow adapters actually can have some loss, I've seen that; but very good ones don't. Stay away from Chinese/Taiwanese adapters and stick with Amphenol, Kings, Delta, Trompeter, et al. (U.S. made mil-spec ones) and you'll be fine.

    There are type N's that are good to 12 GHz and above. Those are usually the kind I use. The difference between those and "regular" N's is they are machined more precisely, often from gold plated nickel flashed stainless steel, which can be machined to closer tolerances than brass can.

    At 70cm, you'd never see the difference.
  5. KM1H

    KM1H Ham Member QRZ Page

    Its all additive as I keep stating. There will be some loss and even used mil spec could be questionable. Id go with new matching crimp connectors for the LMR and just hope the elbows are good.

    If you can, bench test it all from the radio to antenna connector.

  6. WA7PRC

    WA7PRC Ham Member QRZ Page

    The best advice in this thread. Changing a POS connector at the rig end of the feedline is a minor PITA. Changing a POS connector at the antenna end of the feedline is a major PITA.
  7. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    The "bench test" is a wonderful idea that most probably don't follow.

    It's easy for me, I just go to the shop and use a signal generator and microwatt meter, so I can tell if anything has 0.05 dB loss in a minute or two, and can test at any frequency from 600 kHz to about 6 GHz just by turning a few knobs.

    I never install a transmission line without bench testing it first, with the coil of cable laying on the shop floor and the ends on the bench.

    But then I've seen my own "bench tested" cables go horribly bad in the hands of an inexperienced installer who won't simply unroll a coil of coax properly, and instead leaves the roll of cable on the ground and "pulls" on it, creating terrible kinks in the cable as he pulls.

    To this day, I still show guys at Field Day how to unroll coax, and then how to roll it back up. Seems simple, and when most see it, it dawns on them: Hey, that's how I roll up water hose!

    Gee, ya think?
  8. N0AZZ

    N0AZZ Ham Member QRZ Page

    I just read a long technical article on this very subject now for the life of me I can't remember where maybe someone can refresh me. It dealt with 90* connectors @ VHF and up and the loss that came with them but now I can't remember if it was 259's or n' connectors.

    It made me rethink my install of of the new 2m M2 5 WL antenna with a mast mounted preamp I had 5 90* connectors in the shack, Power/SWR meter in/out, Amp in/out. FT-847 in. This was a lot of loss period of a weak signal setup but was ok for the old antenna but was to much loss for this antenna/radio.

    I redid the operating position for that station so I was able to use LMR-400UF for jumpers behind all the equipment
  9. W8JI

    W8JI Ham Member QRZ Page

    First, there are two types of loss. One is a real loss that dissipates power, the other is a mismatch loss that only increases SWR. We have to be careful in any test to understand what we are doing and sort the two out. If the rig has no internal matching adjustment and is centered on 50 ohms, mismatch loss comes into play.

    Measuring losses requires measuring heat or knowing the real line input power. The real line input power is forward power - reflected power. The real load power is forward power minus load power. To be accurate the same test meter should be moved from one end of the line to the other, and second source-reference power meter left at the source.

    What I do here is install connectors and high pot the cable, and do a DC resistance check. This is the single most telling test, much better than any SWR or loss test. A good UHF connector on any cable, from RG58 through RG8 sizes, will hold off at least 5000 volts. Any arc will be from pin to inside of the shell over the face of the connector.

    SWR tests and loss tests are OK, but they are not the ultimate authority. SWR or loss will tell us if something is possibly good, but will NOT really tell us if a connector has been installed correctly and will be reliable.

    High potting tells us if the connector has been installed correctly and cleanly and gives us a good idea if the connection will be reliable. If you melt the dielectric, leave a stray strand loose, or have the braid too close to the center it shows in a high pot test. Problems like that, unless they form a complete short, do not show in SWR or loss tests.

    For connection resistance (bad joints) an ohmmeter is as good as a loss measurement. Just be sure to wiggle the cable while checking.

    The best test of course is both, but when we make new cables up here we only ohm and HV breakdown test them. This is because it is HF through low VHF and losses in connectors are pretty much immeasurable anyway in the overall loss of a long transmission line.

    As for weak signals and accumulations of joints or connectors, remember 1 dB of loss is NOT a change of 1 dB in system signal-to-noise. As a matter of fact with normal antenna and receiver noise temperatures, an additional 1 dB feedline loss is a fraction of a dB signal-to-noise change.

    What is also critical, when mixing multiple mismatched connectors in transmission lines, is the distance between connectors. The worse case situation is when they are an odd quarter wave apart, and the best case when they are a multiple of 1/2 wave apart.

    73 Tom
  10. KM1H

    KM1H Ham Member QRZ Page

    A 1dB loss before the first RF amplifier increases that stages noise figure by 1dB, all the gain in the world will not erase that loss. One good test of system performance is to point the antenna at the quietest spot on the horizon and record the backround noise level. Then point at the sun; you better have a large noise increase. Finally aim at a cold point in the sky and the noise should be below the horizon test, if so you have an adequate installation for very weak signal work and likely even some EME.

    Not everyone has a mast mounted preamp where the loss can still be substantial at UHF and above in some arrays with poor connectors and cables. When the preamp is in the shack whatever the total feedline and connector losses are added to the noise figure. Nothing amazes me more than the ham who spends kilobucks on equipment and then searches for the cheapest deal on coax which is usually a no name or private label rip-off of a quality brand.

    SWR loss is real and should be known and evaluated; it may be ignored in some cases such as using 50 Ohm N connectors on 75 Ohm cable when ultimate performance is not the goal or the frequency is low enough.

    Athother way to evaluate your complete feed system is with a TDR which is a part of many service monitors that work up to UHF. These are becoming quite reasonable on the used market as 2 way shops fold up or newer equipment with higher frequency ranges become needed. The TDR does not replace the power test, it just shows where a potential problem is located. I use a TDR several times a year on all feedlines including HF and keep a photo record.

  11. W8JI

    W8JI Ham Member QRZ Page

    That is true, but the overall impact of a 1 dB "front end" NF increase on S/N ratio is very small unless the antenna is pointed into space or the system had no headroom to start with.

    For example the composite noise figure of a system with 6 dB noise figure from the antenna, -2 dB feedline loss, a 1.5 dB NF preamp with 10 dB gain, and 6 dB NF mixer into a 1 dB NF IF amp is about 7.1 dB.

    That composite noise figure goes to 7.3 dB, a 0.2 dB increase, when the feedline goes from 2 dB loss to 3 dB dB loss.

    If feedline losses are fully establishing the noise figure of the system, a 1 dB increase in feedline loss would increase system composite noise figure by 1 dB, but very few systems are that bad.

    It is the overall noise figure change that matters, not the NF change at one point in the system.

    73 Tom
  12. KM1H

    KM1H Ham Member QRZ Page

    With the average all band rice box the 70cm front end is already noiser than the antenna temperature. Any additional loss is added to the system NF which just makes it into a real alligator....all mouth and no ears:( which reminds me of a lot of many 160M wannabe DX chasers:mad:

    And with digital modes any loss is bad.

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