1/2 wavelength coax line length myth

[KOA4705]

I noticed the trend of people using Bird model 43 watt meters. Also people using a specific length ( 1/2 wave length) of coax for their setup. So I took a look at the Bird model 43 manual, 920-43 page 17. Here is what I found:

RF Directional Thruline® Wattmeter Model 43 Load Matching

When a Bird 43 is used to tune a load to a transmitter and a good match is obtained, removing the unit will not change the match quality. A good 50 ohm load can terminate a 50 ohm transmission line of any length without altering conditions at the transmitter. The 43 is just an extra length of 50 ohm line in series with the measurement.When the load is not well matched (an antenna with a VSWR of 1.5 or 2.0) the line length between the load and the transmitter will transform the load impedance as seen at the transmitter. Removing the wattmeter shortens the
total line length by four inches plus two connectors. This is still not significant at low frequencies where five inches is a small fraction of a wavelength, but at higher frequencies the frequency or power output of the transmitter may be affected. Transmission line theory shows that if the line length changes by exactly wavelength, the impedance is unchanged. To have identical match with the wattmeter in or out of the circuit, insert or remove 1/2 wavelength of line (including the unit). To do this, use a length of cable which, when added to the unit,
equals a 1/2 wavelength at the frequency of interest. If multiple frequencies are needed, a separate cable length is required for each.


This has added even more confusion because there is a caveat in the manual text "Load Matching". For example lets say the antenna feed point is not 50 ohms. A length coax is connected to the antenna/ load. At the other end of the coax a matching network is used to transpose to 50 ohms. At the 50 ohm matching point the coax line length feeding the transmitter is irrelevant. The transmitter will be presented with 50 ohms with any random length of line. But if a random length of coax is inserted between the antenna and matching network (matched load) this is the problem. The impedance presented to the matching network will change. If line length isn't 1/2 wave or a multiple of that. The matching network will not transpose the end of the coax to 50 ohms.

[The DB]

I am curious why, on a single band antenna, you would put coax between an antenna and a matching network? Sure, it can be done, but you are just needlessly adding to the complexity of your antenna setup, not to mention adding in additional losses as more of the signal now has to travel over a lossy medium...

Also, if you are using a Bird watt meter to measure forward and reflected power to calculate SWR, you don't need to limit yourself to an electrical half wavelength of coax as, unlike impedance, SWR does not change over the length of coax, unless you have a problem.


The DB

[MDYoungblood]

Interesting and I guess if it is recommended by Bird, it must be accurate. I personally use an antenna analyzer to adjust antennas now, never owned a Bird meter but have a good Daiwa inline just to watch the needle swinging.
I've never used multiples of coax length for any of my antenna, just used long enough to go from "A" to "B" and a little wiggle room for moving thing around, same goes for jumpers between meters and radios. Now there are some antennas out there that in the instructions require a given length of coax plus show coax types and their velocity factors.

3's

Greg

[Bluerunner]

A No ground plane antenna may benefit from a particular coax length because of the CMC flowing on the outside of the coax inherent to that antenna design. The length can be calculated to cause a CMC wave to be at a null at the transmitter end.

When calculating coax length in regards to CMC on the SURFACE of coax a velocity factor of around 99-96%, closer to that used for calculating a resonant wire antenna length, can be used. In regards to CMC on the surface of the coax, the internal velocity factor of the coax is irrelevant.

CMC is sneaky and can make your meters lie to you. This may account for the perceived improvement in SWR with different coax lengths. Whether the improvement is actual or a consequence of a satisfying CMC lie can't be determined until a measurement is made with any CMC currents choked off.

A good 50 ohm antenna load doesn't care what coax length is used, as long as there is no CMC complicating things.
Tuning up a homemade antenna might benefit from a 1/2 wave multiple feed line if you are starting with a high SWR that may be transposed by the coax length and cause erratic readings.

Antenna currents, CMC, can flow on the outside of coax with many antenna designs, less likely with ground plane antennas but possible as varied surroundings can affect any antennas performance.

A coax coil choke is way too easy to use to choke CMC currents off of the feed line and eliminate potential errors.
I never trust any readings unless there is at least one choke coil in the feed line.

I have often wondered about recommended feed line lengths from antenna manufactures. I wonder if antenna currents radiating on the feed line are engineered into the design and are necessary to get the advertised radiation patterns and take off angles and also perhaps put a CMC null at the transmitter due to feed line length.

Maybe somebody handy with an antenna program can see how antenna currents on the feed line (as an extension of the antenna element maybe) may affect radiation patterns. I expect the program would default to ignoring antenna currents on a feed line.

Just my 2 cents.

[The DB]

A No ground plane antenna may benefit from a particular coax length because of the CMC flowing on the outside of the coax inherent to that antenna design. The length can be calculated to cause a CMC wave to be at a null at the transmitter end.

When calculating coax length in regards to CMC on the SURFACE of coax a velocity factor of around 99-96%, closer to that used for calculating a resonant wire antenna length, can be used. In regards to CMC on the surface of the coax, the internal velocity factor of the coax is irrelevant.

CMC is sneaky and can make your meters lie to you. This may account for the perceived improvement in SWR with different coax lengths. Whether the improvement is actual or a consequence of a satisfying CMC lie can't be determined until a measurement is made with any CMC currents choked off.

CMC's are one of the potential problems I mentioned above. Ground loops would be another potential problems that can cause this effect, and with a ground loop, changing the length of the feed line can also appear to let you "tune" said antenna as well. Although, this is less common of a problem than CMC's.

A good 50 ohm antenna load doesn't care what coax length is used, as long as there is no CMC complicating things.

If you don't have CMC issues or a ground loop problem, its not just 50 ohms that will do that. A 1.5 SWR will be 1.5 SWR at the other end of said coax as well. SWR does not change with coax length, at least on its own.

Tuning up a homemade antenna might benefit from a 1/2 wave multiple feed line if you are starting with a high SWR that may be transposed by the coax length and cause erratic readings.

A 1/2 wavelength multiple (and I mean CMC velocity factor, not internal velocity factor) will, for the most part, also have a choke like effect on currents on the feed line for any current fed antenna. If, however, you have an end fed 1/2 wavelength antenna with no ground plane, this length becomes a worst case scenario.

I have often wondered about recommended feed line lengths from antenna manufactures. I wonder if antenna currents radiating on the feed line are engineered into the design and are necessary to get the advertised radiation patterns and take off angles and also perhaps put a CMC null at the transmitter due to feed line length.

There are some antennas that are this way, although in most cases anything that can radiate from a feed line can also radiate from a mast. The Astroplane, for example, requires a certain length mast/feed line length to work at its best. There are also some antenna designs I have seen in older engineering texts that were this way. For most antennas that we have now, that isn't the case, even those many people swear otherwise.

Maybe somebody handy with an antenna program can see how antenna currents on the feed line (as an extension of the antenna element maybe) may affect radiation patterns. I expect the program would default to ignoring antenna currents on a feed line.

I have done such a study in the past, unfortunately I have lost all of my old models, so I would have to model them again. But essentially, RF follows the path of least resistance. If the path of least resistance is a ground plane, that is where the RF goes. If the path of least resistance is the shielding of the coax, well, you get the picture.

Ground planes are a very good way to deal with the path of least resistance. You would have to be one unlucky ...person (to avoid being censored, but I would personally use another word here)... to have a feed line length that has a lower resistance than a ground plane.

However, if the antenna doesn't have a ground plane, or the ground plane is inadequate (like the chassis of most cars), you will have a much higher chance of getting such unlucky length of coax simply because in that case there are more unlucky lengths.

A choke is a good way to deal with the rest, but even here where you place the choke is not universal. On 1/4 and 5/8 wavelength antennas, at the feed point is good, but on 1/2 wavelength antennas without a ground plane, the feed point is the worst place to put it. As far as electrical length is concerned, the IMAX 2000 should also be treated as a half wavelength antenna, for reasons that go far beyond this post.

And as I mentioned the Astroplane as an antenna that uses the mast, in that case about 9 feed below the hoop is the best place, and if possible you should also isolate the mast at the point as well.


The DB

[KOA4705]

I am curious why, on a single band antenna, you would put coax between an antenna and a matching network? Sure, it can be done, but you are just needlessly adding to the complexity of your antenna setup, not to mention adding in additional losses as more of the signal now has to travel over a lossy medium...

Also, if you are using a Bird watt meter to measure forward and reflected power to calculate SWR, you don't need to limit yourself to an electrical half wavelength of coax as, unlike impedance, SWR does not change over the length of coax, unless you have a problem.


The DB

DB that's was just hypothetical example. To try to rationalize why the Bird 43 manual text is requiring 1/2 wave length of transmission line. I remember a matching scheme using a 1/4 wave length of 75 ohm line to match a Quad with a feed point Z of 100 ohms.

[KOA4705]

Interesting and I guess if it is recommended by Bird, it must be accurate. I personally use an antenna analyzer to adjust antennas now, never owned a Bird meter but have a good Daiwa inline just to watch the needle swinging.
I've never used multiples of coax length for any of my antenna, just used long enough to go from "A" to "B" and a little wiggle room for moving thing around, same goes for jumpers between meters and radios. Now there are some antennas out there that in the instructions require a given length of coax plus show coax types and their velocity factors.

3's

Greg

Greg I do the same thing, line length from A to B.

[KOA4705]

A No ground plane antenna may benefit from a particular coax length because of the CMC flowing on the outside of the coax inherent to that antenna design. The length can be calculated to cause a CMC wave to be at a null at the transmitter end.

When calculating coax length in regards to CMC on the SURFACE of coax a velocity factor of around 99-96%, closer to that used for calculating a resonant wire antenna length, can be used. In regards to CMC on the surface of the coax, the internal velocity factor of the coax is irrelevant.

CMC is sneaky and can make your meters lie to you. This may account for the perceived improvement in SWR with different coax lengths. Whether the improvement is actual or a consequence of a satisfying CMC lie can't be determined until a measurement is made with any CMC currents choked off.

A good 50 ohm antenna load doesn't care what coax length is used, as long as there is no CMC complicating things.
Tuning up a homemade antenna might benefit from a 1/2 wave multiple feed line if you are starting with a high SWR that may be transposed by the coax length and cause erratic readings.

Antenna currents, CMC, can flow on the outside of coax with many antenna designs, less likely with ground plane antennas but possible as varied surroundings can affect any antennas performance.

A coax coil choke is way too easy to use to choke CMC currents off of the feed line and eliminate potential errors.
I never trust any readings unless there is at least one choke coil in the feed line.

I have often wondered about recommended feed line lengths from antenna manufactures. I wonder if antenna currents radiating on the feed line are engineered into the design and are necessary to get the advertised radiation patterns and take off angles and also perhaps put a CMC null at the transmitter due to feed line length.

Maybe somebody handy with an antenna program can see how antenna currents on the feed line (as an extension of the antenna element maybe) may affect radiation patterns. I expect the program would default to ignoring antenna currents on a feed line.

Just my 2 cents.

Bluerunner you nailed it CMC issues. Makes sense that measurements of power/VSWR would be inaccurate.

[KOA4705]

A No ground plane antenna may benefit from a particular coax length because of the CMC flowing on the outside of the coax inherent to that antenna design. The length can be calculated to cause a CMC wave to be at a null at the transmitter end.

When calculating coax length in regards to CMC on the SURFACE of coax a velocity factor of around 99-96%, closer to that used for calculating a resonant wire antenna length, can be used. In regards to CMC on the surface of the coax, the internal velocity factor of the coax is irrelevant.

CMC is sneaky and can make your meters lie to you. This may account for the perceived improvement in SWR with different coax lengths. Whether the improvement is actual or a consequence of a satisfying CMC lie can't be determined until a measurement is made with any CMC currents choked off.

CMC's are one of the potential problems I mentioned above. Ground loops would be another potential problems that can cause this effect, and with a ground loop, changing the length of the feed line can also appear to let you "tune" said antenna as well. Although, this is less common of a problem than CMC's.

A good 50 ohm antenna load doesn't care what coax length is used, as long as there is no CMC complicating things.

If you don't have CMC issues or a ground loop problem, its not just 50 ohms that will do that. A 1.5 SWR will be 1.5 SWR at the other end of said coax as well. SWR does not change with coax length, at least on its own.

Tuning up a homemade antenna might benefit from a 1/2 wave multiple feed line if you are starting with a high SWR that may be transposed by the coax length and cause erratic readings.

A 1/2 wavelength multiple (and I mean CMC velocity factor, not internal velocity factor) will, for the most part, also have a choke like effect on currents on the feed line for any current fed antenna. If, however, you have an end fed 1/2 wavelength antenna with no ground plane, this length becomes a worst case scenario.

I have often wondered about recommended feed line lengths from antenna manufactures. I wonder if antenna currents radiating on the feed line are engineered into the design and are necessary to get the advertised radiation patterns and take off angles and also perhaps put a CMC null at the transmitter due to feed line length.

There are some antennas that are this way, although in most cases anything that can radiate from a feed line can also radiate from a mast. The Astroplane, for example, requires a certain length mast/feed line length to work at its best. There are also some antenna designs I have seen in older engineering texts that were this way. For most antennas that we have now, that isn't the case, even those many people swear otherwise.

Maybe somebody handy with an antenna program can see how antenna currents on the feed line (as an extension of the antenna element maybe) may affect radiation patterns. I expect the program would default to ignoring antenna currents on a feed line.

I have done such a study in the past, unfortunately I have lost all of my old models, so I would have to model them again. But essentially, RF follows the path of least resistance. If the path of least resistance is a ground plane, that is where the RF goes. If the path of least resistance is the shielding of the coax, well, you get the picture.

Ground planes are a very good way to deal with the path of least resistance. You would have to be one unlucky ...person (to avoid being censored, but I would personally use another word here)... to have a feed line length that has a lower resistance than a ground plane.

However, if the antenna doesn't have a ground plane, or the ground plane is inadequate (like the chassis of most cars), you will have a much higher chance of getting such unlucky length of coax simply because in that case there are more unlucky lengths.

A choke is a good way to deal with the rest, but even here where you place the choke is not universal. On 1/4 and 5/8 wavelength antennas, at the feed point is good, but on 1/2 wavelength antennas without a ground plane, the feed point is the worst place to put it. As far as electrical length is concerned, the IMAX 2000 should also be treated as a half wavelength antenna, for reasons that go far beyond this post.

And as I mentioned the Astroplane as an antenna that uses the mast, in that case about 9 feed below the hoop is the best place, and if possible you should also isolate the mast at the point as well.


The DB

All Good points!

[warlock35po]

Hello what you need for correct s.w.r.readings at the xcvr end of things is The odd multiple of a Half wave length of the proper coax.The length to be determined by the Velocity factor of that particular coax.

---

We have merged posts.


Added in 19 hours 3 minutes 50 seconds:
Bottom line "antenna Tuners" do not "tune " your Antenna, you have to resonate the antenna by making sure it is the correct length.You also need to follow the odd multiple of a half wave length of coax at your operating freqs.Random lengths will not gave correct readings."Tuners"only fool your xcvr.into believing it is seeing a resonant load, which is not an efficient power transfer just looks good on a s.w.r./watt meter.

[The DB]

Assume everything I am referring to below is electrical, not physical.

Hello what you need for correct s.w.r.readings at the xcvr end of things is The odd multiple of a Half wave length of the proper coax.The length to be determined by the Velocity factor of that particular coax.

Odd multiple of a half wavelength of coax? When it comes to half wavelengths of coax the number of multiples doesn't matter. For quarter wavelengths of coax, however, weather it is an odd or even number does matter. I'm curious as to what difference you think odd and even half wavelength multiples of coax makes?

Bottom line "antenna Tuners" do not "tune " your Antenna, you have to resonate the antenna by making sure it is the correct length.You also need to follow the odd multiple of a half wave length of coax at your operating freqs.Random lengths will not gave correct readings."Tuners"only fool your xcvr.into believing it is seeing a resonant load, which is not an efficient power transfer just looks good on a s.w.r./watt meter.

Here is a video demonstrating this to be incorrect. Skip ahead to 41:13 to get to the exact point I am referring to.

YOUTUBE_ADMIN

YOUTUBE_ADMIN

This is a good video that demonstrates almost every aspect of feed lines and a lot about antennas as well.


The DB

[warlock35po]

I have said all I have to say on the subject and I respect your position.I will leave it there.Regards,warlock35po

[KOA4705]

Hello what you need for correct s.w.r.readings at the xcvr end of things is The odd multiple of a Half wave length of the proper coax.The length to be determined by the Velocity factor of that particular coax.

---

We have merged posts.


Added in 19 hours 3 minutes 50 seconds:
Bottom line "antenna Tuners" do not "tune " your Antenna, you have to resonate the antenna by making sure it is the correct length.You also need to follow the odd multiple of a half wave length of coax at your operating freqs.Random lengths will not gave correct readings."Tuners"only fool your xcvr.into believing it is seeing a resonant load, which is not an efficient power transfer just looks good on a s.w.r./watt meter.

Any order of a half wavelength of line the impedance will be the same as the load. An antenna tuner can transpose the impedance of the line to 50 ohms. Using a antenna tuner at the end of the line doesn't change the transmission line standing wave. Matching a antenna at the feed point will minimize line losses.

---

We have merged posts.


Added in 1 minute 53 seconds:

Assume everything I am referring to below is electrical, not physical.

Hello what you need for correct s.w.r.readings at the xcvr end of things is The odd multiple of a Half wave length of the proper coax.The length to be determined by the Velocity factor of that particular coax.

Odd multiple of a half wavelength of coax? When it comes to half wavelengths of coax the number of multiples doesn't matter. For quarter wavelengths of coax, however, weather it is an odd or even number does matter. I'm curious as to what difference you think odd and even half wavelength multiples of coax makes?

Bottom line "antenna Tuners" do not "tune " your Antenna, you have to resonate the antenna by making sure it is the correct length.You also need to follow the odd multiple of a half wave length of coax at your operating freqs.Random lengths will not gave correct readings."Tuners"only fool your xcvr.into believing it is seeing a resonant load, which is not an efficient power transfer just looks good on a s.w.r./watt meter.

Here is a video demonstrating this to be incorrect. Skip ahead to 41:13 to get to the exact point I am referring to.

YOUTUBE_ADMIN

YOUTUBE_ADMIN

This is a good video that demonstrates almost every aspect of feed lines and a lot about antennas as well.


The DB

Nice video thanks!