loss in coax due to phase

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KK4MIJ, Aug 1, 2020.

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  1. KK4MIJ

    KK4MIJ Ham Member QRZ Page

    Hello all,

    I have what I think may by a unique question. If there are errors in the following due to my misunderstanding or the way I have expressed it, please let me know so that I can learn.

    How do I calculate loss in feed line due to the feed line not being exactly one half wavelength or multiple thereof at the frequency being used?

    After much reading I have learned that a preferred electrical length of feedline is should be a multiple of a half wavelength of the frequency. When this is the case, the signal going to the antenna and then back to the radio is 180 degrees out of phase. The result of this out of phase condition is complete cancelation of any radiated signal from the coax. Also, when the coax is exactly one half wavelength, it has no reactance which implies that that the impedance seen at the opposite end of the coax is exactly the same as the load on the other.

    As an example, I my coax was cut exactly one half wave on for 3.6 megahertz, it would still be a multiple half wave on 7.2 megahertz, and 14.4 megahertz. On the other hand if my coax was cut for 3.8 Megahertz, the corresponding harmonics would be 7.6 megahertz and 15.2 megahertz. As shown, the error could increase with harmonics.

    So the question remains if I had a piece of coax cut for 3.8 megahertz, and operated on the 20 meter band at
    14.3 megahertz, how much signal / power do I loose due to phase in the coax? What is the proper term for what I am looking for?

    Best regards to all,
    Steven
    KK4MIJ
     
  2. WA7ARK

    WA7ARK Ham Member QRZ Page

    Steven, there is no requirement based on science that requires that a normal coax feedline be any particular length, let alone 1/2 wl or multiples there of... Only if coax is intentionally being used as part of a matching network (to transform impedances) is its length critical.

    Unfortunately, there are hams who believe the 1/2wl myth, and there is much misinformation (especially on the web) on that topic..., just as with other topics.

    The loss in coax is determined by three factors: the operating frequency, the coax type, and the SWR that the coax is operated at. Having chosen the frequency, and bought some coax, the SWR that the coax is operated at is a consequence of a mismatch between the characteristic impedance (called Zo) of the coax itself (i.e. 50 Ohms for RG-8X) and the antenna feed-point imedance at the operating frequency.

    Adding to the length of coax decreases the SWR only slightly, and that is due to loss in the coax itself, and this is not the way to improve the efficiency of an antenna system.... The only way to reduce SWR on the coax is to change the feed-point impedance of the antenna itself so as to bring it closer to Zo.
     
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  3. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    ^True, but the "decreasing SWR only slightly" depends on the native loss of the coax and the load mismatch.

    With a 20:1 VSWR, coax loss increases dramatically and the SWR at the transmitter end will be much lower than the native VSWR at the antenna. This is so easy to demonstrate at VHF, where losses are all higher in any kind of transmission line.

    If your line loss is just 3 dB, and that's quite low loss at VHF-UHF, your SWR at the transmitter end of the cable can never be higher than 3:1 regardless what the antenna is, including "no" antenna.:p

    The phase discussion is irrelevant. Coax radiation is due to common mode current which can occur irrespective of line length and is mostly caused by imbalance and line coupling to the radiator. A common dipole connected without a current balun always has a problem as one side of the antenna is common to the coax outer conductor, and that conductor always becomes part of the antenna unless it's "choked" to prevent that.:)
     
    K0UO likes this.
  4. WA7ARK

    WA7ARK Ham Member QRZ Page

    There is a very useful learning tool that shows the concepts I describe in post #2, above. Dan, AC6LA, a frequent contributor to this forum, makes available a free Windows program, called TLDetails.exe (Transmission Line Details). It let's you create what-if scenarios to answer questions like "what happens to SWR if I change the length of my RG8X coax?"

    Say you have a 40m dipole that is cut too long so that instead of it being an ideal 50 Ohms, at 7.2MHz, it is actually 70 + j30 Ohms (equivalent to saying 70 Ohms resistive and 50 Ohms of inductive reactance). It takes an antenna analyzer or a VNA to measure that complex feed-point impedance, but I am using the numbers as an example, and to make a point...

    upload_2020-8-1_16-37-42.png

    I use TLDetails to show happens if the length of coax (RG-8X as selected in Type), is 50ft (selected in Length), at 7.2MHz with the specified 70+j30 load. The program shows us that 50 ft of RG-8X (Velocity Factor=0.82) is actually 0.453 wl long. You can select the length of coax in wavelengths, if you want.

    Notice that for the conditions we specified, the program shows us the R and jX (as seen by the transmitter, Input end) is 49.6 + j26.8, at a Swr50 of 1.7.

    Notice the slightly higher Swr at the load end ( the difference is due to loss in 50ft of RG8X at 7.2MHz), WB2WIK's concerns notwithstanding.

    More to the point, look at the "loss" numbers in the lower right. Notice that the total coax loss in this example is 10.1W out of 100W

    I encourage you to download and use the program interactively. If you do, you can now change the length of the coax, while watching the Swr, input-end impedance, and total coax loss... This is pretty convincing demo that using coax length in an attempt to reduce Swr is a fool's errand...
     
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  5. W1VT

    W1VT Ham Member QRZ Page

    Coax radiation is the result of the skin effect. In other words, the coax shield works really well, so much so that the inside of the coax doesn't know what the outside is doing. So, if you have an unwanted current on the outside of the shield, say from an unbalanced antenna and no balun, there is nothing to cancel out the current on the outside of the shield. The outside of the shield is an antenna element. And easily modeled as as such with EZNEC.

    Zak W1VT
     
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  6. K7TRF

    K7TRF Premium Subscriber QRZ Page

    You're reading the wrong sources.

    The big advantage of using coax that's half a wavelength long at the operating frequency is that the actual impedance at the antenna feed point is shown at the shack end of the coax run. That can be handy for some antenna testing but there is no requirement to use only half wavelength (or integral multiples of half wavelength) transmission lines. Sure longer lines add incremental loss so keeping the line close to the necessary length to reach the shack makes sense but there is no requirement outside of testing purposes or phased array use as described above where you need specific lengths of coax in wavelength terms.
     
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  7. WA7ARK

    WA7ARK Ham Member QRZ Page

    Common-mode is red herring in Steven's original question....
     
  8. KK4MIJ

    KK4MIJ Ham Member QRZ Page

    Thank you all for responding. You have given me much to think about, and more to research.
    But let me pose a theoretical condition.

    1. Given my antenna is resonant (no reactance) and the resistive load is 50 ohms.
    2. If I feed this antenna with a half wavelength of coax, or a quarter wavelength of coax, will my effective radiated power be the same except for the loss (Db / 100 ft.)
    of the difference in coax?

    Reading the ARRL Antenna book, I learn that twin lead will radiate if the electromagnetic fields do not cancel completely because of being out of phase.
    Will Coax radiate under the same condition, or is all of the electromagnetic wave still contained between the outer shield and inner conductor? Is there any heat generated by being out of phase resulting in power loss.?

    Thank you all again.
    Steven
     
  9. WA7ARK

    WA7ARK Ham Member QRZ Page

    In a 50 Ohm, well-matched at the load-end, the coax loss is function of coax type, frequency and coax length.

    Any conductor carrying RF current will radiate. Examples: an intentional antenna wire, coax carrying common-mode current on its shield.

    With two, closely-spaced parallel conductors, the radiation is proportional to the (vector) difference between the currents on the two wires.

    With coax, the currents inside the coax (center conductor and inside of the shield) are always equal but opposite, however, the outside of the coax shield can carry a third current unrelated to the other two...

    I love this image by @GM3SEK:
    [​IMG]
     
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  10. K7TRF

    K7TRF Premium Subscriber QRZ Page

    Yes. The only difference in radiated power between those two scenarios is that the half wavelength coax is twice as long as the quarter wave section so for the same coax type at the same frequency the half wave section will have twice the matched loss.

    In any feed line if the vector sum of the instantaneous AC currents don't cancel at a given point along the feed line then there is common mode current on the feed line. This is true of twin lead or open wire feed line but also true for coax. Unbalanced situations like this occur when the antenna system is not intrinsically balanced or lacks a common mode choke (aka current balun) at the feed point to drive balanced currents into each half of the antenna. Regardless of whether you run twin lead or coax you want an antenna that is balanced from a current perspective at the feed point.

    Don't confuse matching as in a 50 ohm match at the antenna feed point with current balance into the antenna halves. They're not really the same thing. You can have a very well balanced antenna like a Doublet up high and in the clear that is well balanced but presents a very poor natural match to 50 ohm systems on various bands and requires a wide range matching network back in the shack to use with nominally 50 ohm transmitters. Or you can have an antenna like a dipole that presents a very good match to 50 or 75 ohm systems but due to installation issues or unequal element lengths is quite imbalanced and without a good choke at the feed point can lead to high common mode currents on the feed line regardless of whether that feed line is coax or twin lead, ladder line or open wire feed line. How you route the feed line can also contribute to common mode issues, for instance it's best to run the feed line perpendicular to the antenna elements and not parallel to either one.

    But even with common mode on the feed line (regardless of feed line type) the feed line becomes a radiating (and receiving) part of the antenna system. From that perspective power isn't really lost due to common mode currents, it's just radiated on part of the system that isn't intended to be a radiating part of the antenna. Sure you could look at this as less power reaching the part of the antenna system you really care about but the power is still radiated. Some antennas like the so called Carolina Windom are actually designed to have a portion of the coax feed line act as a radiating part of the antenna.

    Bottom line, common mode RF currents can happen on any type of feed line if the antenna itself isn't balanced or your transmission line routing couples RF back onto the feed line. That can cause problems ranging from RF in the shack to excessive noise pickup (for instance if your feed line runs near an electrically noisy DSL line or similar, remember common mode on the transmission line means the feed line acts as both a transmitting and receiving portion of the antenna). Sticking with well balanced antenna designs (e.g. Dipoles, Yagi's Hexbeams, Verticals with good radial systems, etc.) and paying attention to feed line routing can help reduce common mode on your feed lines but a good common mode choke at the feed point is cheap insurance against many common mode problems.
     
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