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RF connector adapter loss

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KC5MO, May 19, 2011.

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  1. KC5MO

    KC5MO Premium Subscriber QRZ Page

    Hey all,
    Is there an average loss ( dB ) for the RF connector adapters such as a PL-259 to BNC?

    Herb KC5MO
  2. KC2UGV

    KC2UGV Ham Member QRZ Page

    They are generally spec'd out on the connector. It's called the "insertion loss". Generally, I've seen around 0.1dB.
  3. G0GQK

    G0GQK Ham Member QRZ Page

    Not so that you would notice on a long distance DX contact, or even a short one !
  4. W8JI

    W8JI Ham Member QRZ Page

    It is immeasurable normally except for thermal measurements.

    There are some commonly propagated false notions, like .1dB, but it is pretty easy to see that is exaggerated.

    .1 dB is just over 2% power loss. That means if we applied 1000 watts, over 20 watts world turn to heat in the lossy area of the connector. That would melt or unsolder the connector in no time. If you run a kilowatt and the connector is not buring hot, you can be sure it does not have near 0.1 dB loss! As a matter of fact if you run only 100 watts and the connector is not warm, it could not have 0.1 dB loss.

    Loss at upper HF is more typically around .01 dB, not .1 dB. This is why it is so difficult to measure loss without doing it by thermal measurement.

    There have been bad connectors, like connectors with steel springs inside or connectors with exceptionally poor dielectrics, but even those are less than .1 dB at HF.

    At upper VHF mismatch (impedance bump) starts to become an issue, but not at HF.

    73 Tom
  5. KC2UGV

    KC2UGV Ham Member QRZ Page

    Thanks for the correct Tom :) I shifted my '0' and my '.'
  6. K9STH

    K9STH Platinum Subscriber Platinum Subscriber QRZ Page

    Most adapters have a very low loss, one that cannot be measured without some pretty good test equipment.

    A number of years ago I ran into a situation with an installation that had used some Radio Shack PL-259 and PL-258 connectors in the commercial 450 MHz band in which each connector had an insertion loss of just over 1 dB per connector! A technician had wired up a test bench using RG8/U coaxial cable and a total of 6 Radio Shack PL-259 connectors and 2 Radio Shack PL-258 "barrel" connectors. On radios which were putting out over 35 watts on my test bench were putting out less than 5 watts on the second bench.

    Using a Bird 6154 "termaline" wattmeter and another Bird 43 I checked out the installation. Using my Bird 43 with the technician's dummy load and with my 6154 the output from the radios was over 35 watts. When my Bird 43 or my 6154 were put in the circuit the power output dropped to around 5 watts. I examined the workmanship and the technician had installed the connectors properly. As each of these connectors was replaced with an Amphenol equivalent the power read on the wattmeters went up right at 1 dB. Finally, when all of the Radio Shack connectors were removed the power output read over 35 watts on all 3 wattmeters.

    The insulating material on all of the Radio Shack connectors was some really "funky" colored material that I had never seen before. Whatever this material was, it was causing a power loss of pretty close to 1 dB. Also, when held in the hand, each of these connectors was definitely warm indicating that there was power being dissipated in the connector.

    I realize that this example was extreme. However, over the years, I have run into various Radio Shack connectors that definitely did have problems although not anywhere near as bad as that particular group. The result is that I absolutely refuse to use any Radio Shack r.f. connectors. Besides, quality connectors are available from places like Mouser for at least as cheap as Radio Shack, and in many cases cheaper than Radio Shack.

    Glen, K9STH
  7. W8JI

    W8JI Ham Member QRZ Page


    I know the material you are talking about, and it was a bad material. My measurements showed that material had about .05 dB loss in a single SO239 at 30 MHz.

    But we need to be careful how we measure things. The problem in that test setup is each connector causes an impedance bump. If we distribute those bumps over a length of transmission line that is more than a few degrees long, the impedance bumps do more than just add. The resulting measurement will never be better than the total of the real loss, and the measurement with a series of parts distributed along a transmission line is with virtual certainty much worse than sum of the real losses in each connector.

    Let me give an example. If we have a connector that causes a 35 ohm bump at one point in a line, and we add another connector causing the same bump 1/4 wave away, we now have the first bump multiplied by the mismatch of the second bump. It is no longer just addition.

    Not only that, we have also increased the transmission line losses in any line sections.

    This is why, when we have a noticeable mismatch in a connector or cable, we need to be careful to not use too many of those connectors or cables alternating throughout the system. Standing waves and transformer effects can really cause some odd effects that mislead us.

    73 Tom
  8. KC5MO

    KC5MO Premium Subscriber QRZ Page

    Thanks for the replies! The reason for the question is that I have a beacon on 28.2275 set for 2W and I was wanting to get a better idea of my loss. I am using 50 feet of RG-213 to a new carolina windom which has a line isolation transformer and a 2.5' length of LMR-240 to a 4:1 balun. At the radio end I have a tuner where I had to add a BNC to PL-259 on each end. The tuner is there mostly to check on TX power at a glance and not really to "tune" the antenna which has a 1.6:1 SWR without the tuner. Not too bad for a attic antenna! I had the parts laying around so I figured I would give it a try.

    Herb KC5MO
  9. N3OX

    N3OX Ham Member QRZ Page

    Couple watts out of a hundred lost in 23 adapters on ten meters:


    This included a few extremely cheap adapters, some plain old phono connectors that were literally pulled from a Radio Shack audio wall plate (gold one), and a variety of others. Some of them were good, like Amphenol silver plated N connectors. Something like 0.004dB per connector in this test, though that won't split equally... plus it's only about double the total loss of the same physical length worth of RG-58.

    I also didn't spend much time sorting out any effects of the impedance of this short length of weird adapter-line, but the SWR viewed through the whole mess was less than 1.3:1 so I doubt that was too important.

  10. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Wait a minute.

    I didn't see the S-band waveguide-to-binding post adapters in there anywhere.
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