Coax length in multiples of 11?

[JayMojave]

Hello PKY and All: My point is that the coax length subject has many functions, many uses, not always used as a point A to point B power transfer Transmission line.

As the many books and such usually hang there hats on a perfect coax and antenna, they leave out quit a bit of real world conditions.

Another problem / consideration is some antennas are poorly designed having common mode currents and this throws many a curve. Flaky SWR meters, intermittent coax and coax connectors and such.

So my complaint to the academic world I say is valid. But at the same time its a technical subject having many many considerations and multiable things that need to be known from experience to measure and tune / fix a antenna installation.

AC9BX said:
Lowest SWR does not mean best performance. It is common for the resonant frequency where ideally the antenna radiates the best to not be an exact match for the feed and radio and have some SWR

This statement is dead on. I have measured two identical antennas sweeping the frequencies and have seen the higher VSWR at a higher frequency have the lowest free space loss indicating the antennas had more gain or a higher efficiently at a higher frequency than there center tuned lowest VSWR.

Point is the VSWR is not "the" dead on measurement, but close. Field Strength measurements are also needed. A little bit more involved. Hope you agree...

Jay in the Great Mojave Desert

 

[prcguy]

To comment further on the best performance at lowest VSWR, that is a topic with a lot of variables. For some types of antennas like a shortened vertical whip with very high Q at resonance (typical mobile CB antenna), if you detune it to say a 5:1 VSWR you will notice an obvious decrease in performance and it will radiate the best when the match is at or near 1.00:1.

On other types of antennas like a half wave center fed dipole you can go from a perfectly matched dipole fed with 50 ohm coax and replace the coax with 600 ohm open wire line which will immediately introduce about a 12:1 VSWR mismatch. The books say you have just caused about 71% of your power to be reflected and the antenna will only radiate about 29% of your signal, which should be obvious to anyone on the receiving end.

But wait, the receiving station says they can't tell ANY difference when you went from the perfectly matched dipole with coax to the 12:1 match with open wire line. What happens in real life is 71% of your power is reflected back to the transmitter or antenna tuner, but it gets reflected right back to the antenna and radiates more, reinforcing the original signal, reflects 71% of what's left back towards the transmitter, gets reflected back to reinforce the radiated signal again and again.

In this case the feedline loss has a lot to do with the end result and 600 ohm open wire line is virtually lossless at CB frequencies, even under extreme mismatch conditions. There is some loss operating with a 12:1 mismatch between the feedline and antenna with this example but its small and would probably go unnoticed to most people. I have not taken into account any losses from placing an antenna tuner in line to match the transmitter to the open wire line but that is usually small.

The same example fed with coax is a different story because the textbook loss spec of coaxial cable is stated for perfect matched conditions. The same 100ft length of RG-8 that might have 1.2dB of loss at CB frequencies under perfect matched conditions could have 6 or 10dB loss under extreme mismatch conditions. In this case its pure attenuation, it absorbs both the main signal heading to the antenna and the reflected signal as it tries to head back to the antenna trying to reinforce the radiated signal.

Take the same dipole working perfectly with coax and lengthen it about 20%. The match might go to 3 or 4:1 and receiving stations may or may not notice a difference but if they do its probably more from coax loss under mismatch conditions than the antenna's ability to radiate. Do the same thing with the dipole fed with open wire line and nobody will notice any change in your signal.
prcguy

Hello PKY and All: My point is that the coax length subject has many functions, many uses, not always used as a point A to point B power transfer Transmission line.

As the many books and such usually hang there hats on a perfect coax and antenna, they leave out quit a bit of real world conditions.

Another problem / consideration is some antennas are poorly designed having common mode currents and this throws many a curve. Flaky SWR meters, intermittent coax and coax connectors and such.

So my complaint to the academic world I say is valid. But at the same time its a technical subject having many many considerations and multiable things that need to be known from experience to measure and tune / fix a antenna installation.

AC9BX said:
Lowest SWR does not mean best performance. It is common for the resonant frequency where ideally the antenna radiates the best to not be an exact match for the feed and radio and have some SWR

This statement is dead on. I have measured two identical antennas sweeping the frequencies and have seen the higher VSWR at a higher frequency have the lowest free space loss indicating the antennas had more gain or a higher efficiently at a higher frequency than there center tuned lowest VSWR.

Point is the VSWR is not "the" dead on measurement, but close. Field Strength measurements are also needed. A little bit more involved. Hope you agree...

Jay in the Great Mojave Desert

 

[Project25_MASTR]

Hello PKY and All: My point is that the coax length subject has many functions, many uses, not always used as a point A to point B power transfer Transmission line.

As the many books and such usually hang there hats on a perfect coax and antenna, they leave out quit a bit of real world conditions.

Another problem / consideration is some antennas are poorly designed having common mode currents and this throws many a curve. Flaky SWR meters, intermittent coax and coax connectors and such.

So my complaint to the academic world I say is valid. But at the same time its a technical subject having many many considerations and multiable things that need to be known from experience to measure and tune / fix a antenna installation.

AC9BX said:
Lowest SWR does not mean best performance. It is common for the resonant frequency where ideally the antenna radiates the best to not be an exact match for the feed and radio and have some SWR

This statement is dead on. I have measured two identical antennas sweeping the frequencies and have seen the higher VSWR at a higher frequency have the lowest free space loss indicating the antennas had more gain or a higher efficiently at a higher frequency than there center tuned lowest VSWR.

Point is the VSWR is not "the" dead on measurement, but close. Field Strength measurements are also needed. A little bit more involved. Hope you agree...

Jay in the Great Mojave Desert

The point was really, the books are out there and do exist. We also know that they work from various techniques taught over the years as well as Motorola's nifty little lowband diplexing chart which they issued with the Syntor X9000's three decades ago.

Now, in 95% of mobile antenna installations, the antenna will be tuned to match the antenna system (feedline and antenna). This renders coaxial matching a mute (moot?) point.

Stations are a bit of a different story however. This is a situation I ran into a few months ago. I was managing a tower crew and we replaced a DB-224 with a new DB-224 due to water intrusion. When the radio shop's tech began testing SWR, he was getting a calculation of 2.03:1 and he thought there was something wrong with the new antenna and/or the installation of said antenna. Where his 7/8" hardline ended he disconnected a super flex jumper running to the repeater's duplexer and inserted his wattmeter. I told him he was measuring his SWR at the union between the superflex and heliax and got him to reconfigure for measuring between his duplexer's antenna port and the superflex jumper. Lo and behold, 1.3:1.

So yes, it does come up occasionally, just not very often with mobile installations where you tune the antenna to the system.

 

[Cruiseomatic]

Stupid question here I know but....
I built a dipole from 2 three foot fiberglass antennas. It would not tune for low SWRs. I didn't have an analyzer to check for impedance's and such. After 2 hours of adjusting the tuning screws I gave up on it. Read about a balun and tried it right at the antenna base. 7 turns tightly packed together in a diameter the size of a 20 oz. bottle.
This time, they were lower and NOT using the coax as part of the system. I was able to tune broadband 1-40 with a 1.2 VSWR with exceptional results giving conditions.

After reading through all this,Would it be safe to always use a balun anyway whether or not the system may or may not need one to prevent doing a lot of work just to find out you need it? Or as a "comfort" knowing ALL the RF is being radiated off the counter-poise and none off the coax as a balun keeps RF from running along the shield?

 

[jonwienke]

I hope I have helped you better understand SWR and antenna systems.

You've contributed more to RF urban legends and misinformation than anything else.

If the electrical length of the feedline is an exact odd multiple of 1/4-wavelength, the phase relationship between the reflected signal and the transmitted signal will fool a SWR meter into showing an erroneously low SWR. But the coax length does not change the efficiency or resonant frequency of the antenna. You can test this by inserting the SWR meter at the antenna end of the coax instead of the transmitter end. You'll get a higher (and more accurate) SWR reading at the antenna end.

The only accurate/useful part of your post was the bit about coiling several turns of coax at the antenna to form a common-mode choke to prevent RF from radiating from the coax shield. That is good advice, especially with J-pole antennas or antennas with non-standard ground planes. As prcguy said, if your coax is radiating RF, the proper fix is to get rid of the common-mode currents so that your RF is being radiated by the antenna instead of the coax. That will get more signal where you actually want it (radiating from the antenna), and reduce interference with nearby electronics. In a vehicle, stray RF from coax can interfere with the vehicle instruments and/or engine control electronics. If you try to mask the problem by tweaking coax length, you won't get maximum performance from the antenna, and you will still have RF radiating from the coax wasting power and potentially interfering with nearby electronics.

 

[swen_out_west]

I really only read the original post from 2 years ago and just shook my head. The guy that started this erroneous value must have thought feet were the same as meters. The actual value that people quote (and sell in CB shops) is 18 feet, which is half of the wavelength (11m/36 feet).

The concept is sound by theory, but not the whole truth. It doesn't even take into consideration velocity factor of each individual.type of coax.

Also, as has been pointed out by many and Jon hit it right on the head, It really only changes the reading of your VSWR and has no bearing on the actual resonant frequency.

As Jon also pointed out a common mode choke is the fix. When I finally worked on making my IMax 2000 as efficient as possible that was the biggest gain in performance. Problem is I made it too efficient, lol. Now I am fighting a stupid transformer substation and high transmission lines an 1/8 mile away. Trying a different filter and hoping that will help. If it doesn't I just give up on talking to barefoot AM radios in town 12 miles away and I'll stick with anybody on this side (finally able to pick up the boys down by Jay (weak but readable, might try 10 more feet someday)) and skip.