adding coax lowers SWR reading on the meter?

[wavrider]

This trick has worked for me for years, using a field strength meter, putting the FS meter against the coax feeding your antenna, you will be able to find a null, cut the coax there an put a coax connector on it, the swr will be the same as what you will see at the base of the antenna, problem solved radio see's a 50 ohm load on that frequency!........KB6HRT

Measure a half wl times velocity factor of the coax, cut it and use that 1/2 wl to tune the antenna, then put whatever length of coax you need to get back to the shack. Easy enough.

This will work for mono band antennas, but multi band, now that would be a lot of pieces of coax laying around.

Interesting idea using the field strength meter, I do not think the 259B analyzer will put out enough RF for that field strength meter to even detect a null.

 

[walterb]

Wavrider
Sometimes if the wattage of the radio has a low carrier 1 or 2 watts you can take a piece of small wire take a couple of turns around the coax then around the FS antenna an you will see a stronger reading on the FS meter, on a HF rig it not necessary most of the time!......KB6HRT

 

[hookedon6]

at the feed point of a dipole theres a curent peak but a voltage null and at thew ends of the dipole its the opoosite but thtas 90 degrees out of phaze not 180 so how can there be a 180 degree null point for both if they are only 90 out of phaze? something has to be high. iether the curent or the voltage

we are talking about a tuned feed line,....
unless the feedline is DESIGNED to radiate, a feedline goes from the transmitter to the feedpoint only,.....

it doesn't care or "know" what type antenna follows the feedpoint

btw: your example of voltages/currents at the ends of a di-pole is not correct,................ it varies with the dimensions of the antenna

 

[Needle Bender]

we are talking about a tuned feed line,....
unless the feedline is DESIGNED to radiate, a feedline goes from the transmitter to the feedpoint only,.....

it doesn't care or "know" what type antenna follows the feedpoint

btw: your example of voltages/currents at the ends of a di-pole is not correct,................ it varies with the dimensions of the antenna

i dont think were comunicating here. i dont thing were talking about the same thing.
i thought the rf curent and voltage in the coax or in the antenena is always 90 degrees out of phaze from each other but you mke it sound like there 180 out of phaze. and im not wrong about the voltage and curent on a 1/2 wave dipole. i didnt think i had to say 1/2 wave dipole. they are almost always a 1/2 wave except when a different length is specified. but i like your idea of feeding at a 1/2 wave high voltage point instead of a 1/4 wave high curent point. i wonder if this isnt one reason why some antennas dont work as well for some people as they do for others. maybe there feeding them at a curent node and its screwing with the match or radiation point for curent or toa

sorry wrong posting pse ignore.

come on henry chime in

 

[wavrider]

we are talking about a tuned feed line,....
unless the feedline is DESIGNED to radiate, a feedline goes from the transmitter to the feedpoint only,.....

it doesn't care or "know" what type antenna follows the feedpoint

Agree and very good point.

OP has to decide what they are actually doing. Are they "tuning" the feedline or tuning the antenna?

If the feedline is designed to radiate, IE G5RV on the frequency it was designed to be used on, the feedline does radiate. Some ops like to use certain lengths of ladderline to "feed" their loops as it makes it easier for the antenna coupler to "tune" the antenna or match impedance.

Basic rule of thumb, cut a half wavelength of coax velocity factor figured in, tune your antenna as the analyzer or swr bridge will not "see" the coax if it is an electrical half wave length, this method is almost certain to allow the op to "tune" his antenna and not his feedline.

Then there is always the ground effect to figure in. If you tune your antenna at 9' off the ground then install it at 70' on the tower, Your VSWR will change, the antenna is no longer coupling to the earth.

Lowband HF antennas installed close to the ground have great broadband width, due to the coupling to earth ground. Get that antenna .25 wl in the air and it will change, so many variables to be considered in tuning an antenna.

It sure is a lot of fine building and learning about it though.

 

[KM3F]

....How about looking at the theroy behind all this for the correct answers.
....If the antenna is the correct match to the feed line (at the operating frequency), the feedline length makes no difference except for loss over it's length.
....If however the antenna is not a match, reflected power returns to the source also with second trip losses on the same feedline..
....Since there is wave length involved, in makes a different where in the line you measure and if you change the feedline length. Because you now have something that is frequency/length sensitive with return power just the same as the antenna demensions are sensitive. The feedline becomes part of the whole match situation depending on where in the line you look at it.
....Also add to this that the farther you get from a mismatched antenna the lower the return loss you will see on a meter because of the longer feedline losses.. So don't let it fool you that the return power ratio looks better. You are just seeing less return power on the meter due to the cable losses.
....It's not considereing the whole theroy for the two different situations that confuses people.
Good luck.

 

[BOOTY MONSTER]

is a MFJ 259B immune to the effect of showing inaccurate readings if a antenna is not perfectly tuned to frequency and the feedline is not a electrical 1/4 WL of that frequency ?

also , if a 259B shows different impedance on a antenna with different coax lengths is that normal ? or is there something wrong with the coax ?

 

[Needle Bender]

-i just cant believe high wattage traveling thru a medium where theres phaze, frequency and velocity doesnt care or make a diference. seems like is part of the circuit but maybe not.
im going to build a antenna in a month and ill try cutting the 213 right at the 1/2 wave multiples and get signal reports then cut it down by a electrical 1/4 wave and test it again for any performence diferences just so i know if it matters or not
booty that thing is twisting my eyes out of there sockets!

 

[Henry HPSD]

QUOTE :
is a MFJ 259B immune to the effect of showing inaccurate readings if a antenna is not perfectly tuned to frequency and the feedline is not a electrical 1/4 WL of that frequency ?

also , if a 259B shows different impedance on a antenna with different coax lengths is that normal ? or is there something wrong with the coax ?
END OF QUOTE.

No a MFJ 259B is not immmune if the feedline is not a electrical 1/4 wl....
Where did you get that 1/4 wl by the way ?
If it were...you wouldnt be able to tune a 2el quad to 50 ohms.

For a correct SWR measurment the cable should be a electrical 1/2 wl or a multiply of that.
NOT a 1/4 wave. SWR repeats it self after each 1/2 wl.

There is only one situatie where it shouldnt mather in reference to a SWR reading.
And thats where the entire system is 1:1. HOWEVER ! it is possible to get that reading without having actually the system 1:1.
Thats why one always should measure at the feedpoint or after that 1/2 wave length.

Yes it is normal that a 259B shows different impdedances with different coax lengths.
If the antenna / radio/ coax are not the same impedance the coax for example will work as a impedance transformer.
That is exactly the same as you do with your quad...
The impedance is around 100/120 ohms...attaching a 1/4 wave 75 ohm cable will provide a good match towards the 50 ohms.

There you already see what is important !
1- antenna impedance
2- coax impedance
3- coax length
4- wanted impedance.


@ Needle bender,
It is not a "given" changing the length will "change" something.
If the antenna system is already 1;1 there will be no difference beside the loss of the additional coax length.
If the system depends on a specific electrical length (for example that 100 + ohms quad) it can cause sever demage if you change a that length.
If the system does not not depend on a electrical length nor is it 1:1 you will get different SWR readings. This does not mean people will always notice a change at the receiving end.

Heck, it is already that irritating...and then we realize...it is not SWR which tells us if the system is working to its best abbility or not...


I hope it helped.

Kind regards,

Henry 19SD348
All about antennas

 

[W5LZ]

"is a MFJ 259B immune to the effect of showing inaccurate readings..."
No, it isn't. It's subject to the same inaccuracies any other device is. But it does give you readings that if understood can certainly make things easier. First, you have to know what it's telling you and why. That '259 can tell you things that a normal SWR meter "know's" absolutely nothing about.

"also , if a 259B shows different impedance on a antenna with different coax lengths is that normal ? or is there something wrong with the coax ?"
Yes, it can only tell you what it 'sees', and if that antenna isn't a 'perfect' load impedance (or close), it can only tell you what the combined impedance of the feed line and antenna are. It has no way of 'separating' those two things. That does not mean that there's something wrong with the coax. It simply means that the impedance of the antenna and the coax are not the same (or close).
One way of 'separating' the two thingys, antenna and feed line, is by using a known length of feed line that will show the same impedance at one end that' is present at the other end. An electrical 1/2 wave length feed line has that characteristic, so by using an electrical 1/2 wave length to measure antenna input impedance means that you can set/adjust the antenna from a point further away from that antenna (so your physical presence doesn't affect the readings you get). {One nice thing about that '259 is that it can tell you what an electrical 1/2 wave length is, you don't have to do any calculating. That's just a part of knowing how the @#$ thing works. The 'bad' part is that you have to know how all this stuff 'fits' together, it isn't intuitive. Gotta flex that muscle between them ears, you know? }
- 'Doc

Oops! Little late with a reply...

 

[Needle Bender]

i noticed that bootys questions are getting more knowledgable since back when he was lectured by mr suburban for not already knowing what he was asking about. cool to see someone else learning too
something just occured to me henry. i could use 75ohm all the way down from the home made 100+ ohmn quad as long as it was cut to odd electrical multipels of a 1/4 wavelength and i should get close to 1:1swr right?
if someone had a antena that was 1.5 swr and 75 ohms because it was out of tune and they used 50 ohm coax cut to odd electrical 1/4 wave multipel then they would still have a 1.5 swr but instead of 75 ohms it would be about 33 ohms now because the 50 ohm would transform it. right?
excelent info http://www.para.org.ph/membersartic...nnas_Matching and Feeding Techniques - 21.pdf

 

[BOOTY MONSTER]

i meant electrical half wave length . my bad
this comment on another forum seemed odd to me is why i asked ....

"In a mobile there is no matching network. The only way to match your impedance ( just as a gamma does ) is to alter the coax length. Mind you swr of the antenna itself will not change, but your amp will me much happier seeing a 50 ohm load, run cooler and less chance of blowing pills. I have found that in most all single antenna installs that I have done 15 ft of 213 was the sweet spot. the wifes pick up has 7.5 ft of 213. mfj read 50 ohms 1.0 swr. same with the work truck with 15 ft. when we had 18 or 12 feet the ohms were roughly near 70. When ohms are 70, x=2 you will not read 1.0 swr on the mfj, even though your antenna will be perfectly tuned. the bird will also show more reflect under the same situation. depending on what coax you use may change length. it depends on what velocity factor the coax is."

thanks for the replies

 

[mackmobile43]

i meant electrical half wave length . my bad
this comment on another forum seemed odd to me is why i asked ....

"In a mobile there is no matching network. The only way to match your impedance ( just as a gamma does ) is to alter the coax length. Mind you swr of the antenna itself will not change, but your amp will me much happier seeing a 50 ohm load, run cooler and less chance of blowing pills. I have found that in most all single antenna installs that I have done 15 ft of 213 was the sweet spot. the wifes pick up has 7.5 ft of 213. mfj read 50 ohms 1.0 swr. same with the work truck with 15 ft. when we had 18 or 12 feet the ohms were roughly near 70. When ohms are 70, x=2 you will not read 1.0 swr on the mfj, even though your antenna will be perfectly tuned. the bird will also show more reflect under the same situation. depending on what coax you use may change length. it depends on what velocity factor the coax is."

thanks for the replies

This is a counterpoise issue and has everything to do with the vehicles matching network which is the ground plane, who ever posted this is an idiot and has no clue what his mfj was showing him.

I had the same issue with my pickup, I had Cowboy 4570 tune my antenna with his mfj and my amps would run nuclear thermal hot so I got to playing with coax lengths and arrived at 21' of rg8u and re tuned my antenna with an swr meter and lowered my reflected wattage and output wattage went up and the amplifier ran normally cool but I had enough common sense to know that when my antenna is mounted to the rear of the bed that it's view of ground plane is compromised.

 

[BOOTY MONSTER]

it was cool breeze

 

[mackmobile43]

it was cool breeze

Figures