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Author Topic: Help me use my MFJ259B analyzer on G5RV  (Read 5165 times)
CHRISDX
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Posts: 244




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« on: June 20, 2013, 08:20:29 PM »

Hi all,

I have a G5RV and use it for an Icom 7000 transceiver as well as several SWL receivers including an old Hallicrafters SX28.

The ladder line stops outside my window and I have approx 12' of RG8X coax feedline coming in.  At the ladder line connection block, I have a few turns of the coax to create a 1:1 choke or current balun (whatever that is).

So I connected the coax to the analyzer and started checking for lowest SWR's here is what I got:

3.45Mhz 2.25 SWR / 28ohms
7.55Mhz 1.9 swr / 50 ohms
13.95Mhz 1.7 swr / 28ohms
19.47Mhz 1 swr  / 50 ohms
24.27Mhz 2.26 swr / 58 ohms
31.01Mhz 1.9swr / 87 ohms
35.41Mhz 1.7swr / 52 ohms
47.22Mhz 1.2swr / 42 ohms
52.36Mhz 3.2 swr / 58ohms


What does this say to me, and what else can I do with this analyzer to learn this antenna?

Thx!



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W9IQ
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Posts: 104




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« Reply #1 on: June 21, 2013, 04:04:16 AM »

Hi Chris,

You didn't mention it but the G5RV requires an antenna tuner to be an effective transmitting antenna. If you are using just for receiving, the tuner is not really needed.

The G5RV design, like most multiband antennas, is a compromise antenna. Unfortunately, many people assume it is some type of miracle antenna and don't understand how it works or what its shortcomings are. So I congratulate use on trying to learn more through measurement and experimentation.

One of the design issues with the G5RV is that the coax coming into the shack often suffers from common mode current. This is the current that flows on the outside of the coax shield and, in this case, is not desirable. Your few turns of coax may help mitigate some of this but it is probably not enough of a choke to be fully effective - particularly for multiband use. You should look at getting a balun to install between the ladder line and the coax if you are going to use this for transmitting. Common mode currents can throw off your SWR readings as well. It would be interesting to carefully record the SWR at various frequencies with you current configuration and then redo the measurements after installing a balun.

The measurements you obtained for low SWR show you the need for an antenna tuner if you are going to transmit into the antenna on the amateur bands. If the G5RV is the original design, it should show fairly low SWR in the 20 meter band without an antenna tuner. Most solid state transceivers are happy if the SWR is 2:1 or less. More measurements in the amateur bands of interest will confirm the need for an antenna tuner as you will probably see an SWR higher than 2:1.

Your antenna analyzer will also show you the resistive and reactive components of the load. But when you are taking this measurement using any type of feedline, the information does not direclty represent the antenna as the feedline can profoundly impact these numbers. If you want to know the resistive and reactive conditions of the antenna at a given frequency, you should either connect the analyzer directly to the antenna or you will need to do some calculations regarding the length and type of feedline(s) between the antenna and the analyzer. This is not trivial math as it involves calculus. There are websites and tools that can assist you with this calculation. The G5RV design compounds the difficulty of the calculation. The SWR measurements on the other hand, are hardly affected at all by relatively short lengths of feedline.

Noting the reactive and resistive components of the load at the feedline end can be helpful to understand if a tuner will successfully bring the antenna system into a (conjugate) match so that the radio sees a 1:1 SWR. Sometimes adding a little more feedline or taking some away is enough to allow a tuner to find a match or avoid a flash over due to high voltage.

- Glenn W9IQ
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CHRISDX
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Posts: 244




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« Reply #2 on: June 21, 2013, 05:48:46 AM »

Hi Glenn, thank you for the reply. Yes I have a General class license and transmit accordingly using an MFJ-945e tuner to match and I can get a sub-2:1 swr on all applicable bands IIRC.

At 14.000 swr is 2.1   21 ohms    R=24   X=6
At 14.350 swr is 5.1   15 ohms    R=9    X=12

I do have RF coming through a nearby subwoofer for my surround system, which is behind the couch and 2 or 3 ft away from the feedline passing by. Is this related to the "common mode" issue you mentioned?

What is an acceptable balun to control common mode current? 
Also what is the importance or relevence of resistive and reactive readings? (I assume they are the R,X readings)

Thank You
Chris
KB1SNJ



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W5DXP
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« Reply #3 on: June 21, 2013, 06:48:11 AM »

I have a G5RV ...

Is it a 102 ft. dipole fed with ~30 ft. of 300 ohm twinlead? Ladder-line?
How high is the dipole? Horizontal or Inv-V?
Does the 12 ft. of coax include the ugly balun or not?
How many turns and diameter of the ugly balun?

For your measurements to be standardized, they need to be made without the coax through a 1:1 current choke looking into the 300 ohm section. Let's take 20m, for instance. EZNEC says the impedance looking into the twinlead on 20m will be about 100 ohms resistive which causes an SWR of 2:1 on the coax. If your coax is 1/4 wavelength long, the impedance looking into the coax will be 25 ohms resistive. If the coax is a little longer than 1/4WL, your system resonant frequency will be out of the 20m band on the low side, as your is. With a random length of coax acting as a tuned feeder in the system, one cannot tell if conditions are normal or not. Of course, one can backtrack the impedances if the length of coax is known but it is easier to standardize the measurements by bypassing the coax. Everything is known about a G5RV at the 300 ohm tuned feeder so make your resonance measurements at that point without the coax. Then we can tell if everything is normal. Here is how the measurements should be standardized so one G5RV can be compared to another one, e.g. using EZNEC.

MFJ-259B---1:1 choke--+====30' 300 ohm twinlead====102' dipole
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W9IQ
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Posts: 104




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« Reply #4 on: June 21, 2013, 06:49:59 AM »

Hi Chris,

Glad to hear that you have a tuner and it is getting the SWR in range for you. That is an essential element for the G5RV antenna.

The RF getting into things in the house could be due to common mode signals but not necessarily. The common mode problem can come from lack of a good balun and it can be exasperated if your antenna lacks symmetry or the feedline does not fall in a perpendicular fashion from the antenna. Lack of symmetry can be caused by unequal lengths of the antenna legs and by proximity to nearby objects such as trees, etc. It is always advisable to try to keep the antenna as symmetrical as possible but the real world conditions often require some compromises.

There are many good balun suppliers out there. Check out DX Engineering as an example. I am sure others will chime in here with their favorites based on their experiences.

You are right that the R and X readings are the two components that make up the impedance of the antenna system at the point of measurement. Moving up and down the cable will show different combinations of R and X but they will still result in the same impedance and thus the same SWR. The R represents the resistance and the X represents the reactance at the point of measurement. The MFJ meter doesn't directly show you if the reactance (X) is capacitive or inductive but this can be deduced through some secondary measurements. Knowing if the reactance is capacitive or inductive can often help determine if the antenna is too long or too short. Knowing the resistance and reactance also makes it possible to design matching networks to achieve a 1:1 SWR or to determine if a tuner would be capable of providing a reasonable SWR.

When an antenna is resonant, the reactance is 0 and there is only a resistive component. Since there is no reactance, the impedance is the same as the resistive component. Some people mistakenly state that the resistance must be 50 ohms to be resonant but this is false. Many antennas exhibit a resonant resistance of other than 50 ohms. This also means it is possible to have a resonant antenna and not obtain a 1:1 SWR with only a 50 ohm feedline system. A typical dipole is a good example of this - if it is resonant and is hung far enough away from the ground, it will exhibit a resonant resistance (and impedance) of close to 70 ohms. Many people unknowingly de-tune their dipole from resonance when they attempt to get a 1:1 SWR with only a 50 ohm feedline.

As you are finding with your G5RV, resonance of the antenna is not a requirement. An antenna tuner can provide bring the antenna system in resonance through the process of conjugate matching . You may find it interesting that it does this by providing the same R value of the antenna system but the opposite sign of the X value of what you measured. While the antenna may not be as efficient as a resonant antenna and you may suffer some signal loss in the feedline, this is an acceptable compromise for many.

If you don't have a copy of the ARRL Antenna Handbook, I highly recommend it as an addition to your book shelf. You don't need to read it cover to cover but you will find it to be an excellent topical reference.

- Glenn W9IQ
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CHRISDX
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Posts: 244




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« Reply #5 on: June 21, 2013, 07:01:01 AM »

I can answer sonme of these now before heading out for work.

It is a standard G5RV. It's actually means a lot to me as a ham buddy who convinced me to get my ticket gave it to me to go with my first transceiver which was a new Icom706Mk2g. A year later I sold that to buy a used IC7000 which works much better than the 706  I might add.

Anyway yes it is an inverted Vee. In fact, I had a tree climber trim my two 70' oaks and while he was up there I had him install a sailboat pulley from west marine with a stainless lag screw in a limb about 50' up over my house. The lot is small so the G5RV legs are not straight 180 degrees but pretty close though.  yes it uses the ladder line about 1.5" wide with the square holes. That hangs down to about 6' off the ground outside the window. The RG8X is coiled a few times about 5" in diameter (no time to go measure now). The 12' of RG8X is an approximate length, might be a little less, just enough to come through the window and reach my 4-way alpha delta antenna switch which feeds the MFJ tune/7000 radio, the Halli SX28, a Drake SW2, and a Degen DE1103 - I like to SWL and compare the radios. The SX28 is king for that, with the SW2 winning when the sync detector is needed to counteract fading conditions. The 7000 has God-awful audio but the DSP can really quell interference at times, and of course I have to transmiit with it which is pretty rarely and on 2m 10m when I do. Very occasionally I will make contact on 40 or 80.

By the way the feedline is soldered at the G5RV junction, I did that to reduce corrosion induced connection problems outside. For storms, I pull the feedline back outside and hang it on the fence post. I bought push-on 259 adapters which I thought would be great for convenience but somehow they dont make the same connection as the threaded connector despite being clean (new). So they are sitting here on the table.

Anyway thanks for the replies, I am finally starting to get into antenna particulars.


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CHRISDX
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Posts: 244




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« Reply #6 on: June 21, 2013, 07:04:30 AM »

Hi Glenn

about to run out to work, but i just want to address symmetry, my antenna is not perfectly straight, but close. It has a bend at the top maybe 170 degrees one leg to the other (guessing) due to myu tiny house lot thats the best we could do. The feedline is also a few degrees off vertical.  But nothing too radical.

I'll read the rest of ur reply later. Thank you for the info.  Smiley

Regards,
Chris
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AC2EU
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Posts: 473


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« Reply #7 on: June 21, 2013, 08:28:08 AM »

Hi all,

I have a G5RV and use it for an Icom 7000 transceiver as well as several SWL receivers including an old Hallicrafters SX28.

The ladder line stops outside my window and I have approx 12' of RG8X coax feedline coming in.  At the ladder line connection block, I have a few turns of the coax to create a 1:1 choke or current balun (whatever that is).

So I connected the coax to the analyzer and started checking for lowest SWR's here is what I got:

3.45Mhz 2.25 SWR / 28ohms
7.55Mhz 1.9 swr / 50 ohms
13.95Mhz 1.7 swr / 28ohms
19.47Mhz 1 swr  / 50 ohms
24.27Mhz 2.26 swr / 58 ohms
31.01Mhz 1.9swr / 87 ohms
35.41Mhz 1.7swr / 52 ohms
47.22Mhz 1.2swr / 42 ohms
52.36Mhz 3.2 swr / 58ohms


What does this say to me, and what else can I do with this analyzer to learn this antenna?

Thx!





What is is saying to you is that the G5RV is a terrible antenna!

It would be more informative if you had posted the complex impedance rather than just the magnitude.
The radiation RESISTANCE is where the "rubber meets the road" which is the first number in the display in the format of
R,X. Ideally, the R=50 and the X=0 for a "perfect" match. This rarely, if ever happens in real world installations, but your antenna will not even approach this condition anywhere in the ham bands.
OK, I am now putting on my flame retardant suit to defend against the G5RV high priest assault.  Grin

BTW, you can put the polarity signs to the X value by increasing the frequency slightly while observing the X change.
if the X decreases with a frequency increase, the reatance is capacitive with a negative sign.
If it increases with frequency then it is inductive indicated by a positive sign.
Now that you know how to get the most from your analyzer, you can build and optimize a good antenna.  Grin
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W9IQ
Member

Posts: 104




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« Reply #8 on: June 21, 2013, 11:02:08 AM »

Jim,

I don't think it is objective to say that the numbers Chris posted shows it to be a terrible antenna. If this were treated as a black box experiment and all you knew was that these are the readings at the end of a piece of coax that is connected to an antenna, you wouldn't be able to reliably conclude that the coax is connected to a terrible antenna.

Knowing the R and X values for each Z would not have added much useful information since these again are at the end of the coax and not readings from directly at the antenna. A small change in feedline length would likely have a dramatic change on the R and X values but would not change the Z provided there is no common mode problem.

I can surmise you are not a G5RV fan but perhaps I am missing some other point?

- Glenn W9IQ
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W5DXP
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« Reply #9 on: June 21, 2013, 11:49:40 AM »

What is is saying to you is that the G5RV is a terrible antenna!

Sorry, that's only if one believes in black magic. It is actually saying that the G5RV obeys the laws of physics. I have already pointed out what is conceptually wrong with those irrelevant measurements. On the bands where the G5RV works well, it is simply a dipole with a series matching section producing relatively low measurable losses. On the bands where the G5RV doesn't work well, the series section becomes a mismatching section with resulting high measurable losses. There is absolutely nothing magical, either good or bad, about a G5RV antenna. These Smith Charts show why the G5RV is a pretty good antenna on 80m and 40m plus a chart that shows the optimum series matching section for each particular HF band.

http://www.w5dxp.com/G5RV.HTM
« Last Edit: June 21, 2013, 11:56:31 AM by W5DXP » Logged
CHRISDX
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Posts: 244




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« Reply #10 on: June 21, 2013, 08:08:55 PM »

OK I did make one error in my description, the antenna has a much sharper angle between the legs than 170 degrees, its more like (again, guess but from a better angle) 110-120 degrees, almost an expanded L shape.

As to the measurements, since I have a tuner, does it matter to how much actual power is radiated of my radios 100watts?

Also, I will take more measurements at the antenna using a 12" patch to the analyzer.

Did you guys need me to post the X R values as well as lowest SWR points and meter ohms?

The original question of course is how to make use of this analyzer and possibly make changes to my antenna/feedline based on what it reports. (and learn something in the process)
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AC2EU
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« Reply #11 on: June 21, 2013, 08:30:30 PM »

What is is saying to you is that the G5RV is a terrible antenna!

These Smith Charts show why the G5RV is a pretty good antenna on 80m and 40m plus a chart that shows the optimum series matching section for each particular HF band.

http://www.w5dxp.com/G5RV.HTM


DXP, one the G5RV high priests has shown up on cue to defend the antenna's honor with more snake oil so that we may be anointed and properly inducted into the cult. ( read the text with the smith chart

The best you can say is that is is "pretty good" on 80 and 40 is because there is no such thing as a multiband antenna unless there is some kind of special electronics associated with it, such as traps.
As you say, the laws of physics tell us that there are very specific conditions for resonance and efficiency. The G5RV may perform like an OCF on the band it's cut for, but all other bands will need an EXTERNAL tuner because engineers realized that designing internal tuners for SWR >3:1 was a waste of money.  
They are implying that you should use a better antenna setup... and you should...

The G5RV is a dinosaur from the days of tube finals that had a broader tuning range. You had to retune those rigs every 50KHZ or so anyway, so nobody knew if it was the antenna or rig. Now we do.
I fail to understand why anyone would want to use a non resonant antenna that needs so much "attention" to get any kind of performance out of it. Why complicate your life?  Shocked

You will be better off with a standard 80M dipole for the band and it will also do a decent job of harmonic tuning on some others - with an INTERNAL tuner!  Grin

Just to be clear, no, I am NOT a G5RV fan.  Cheesy


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CHRISDX
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Posts: 244




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« Reply #12 on: June 21, 2013, 08:42:09 PM »

As to operating it and being fairly new to the hobby, I can say the antenna has served from 80-10m. Even on 10m, I switched between that and a vertical and the report at the other end was the same. I usually can tune it with my manual tuner pretty quickly to under 2:1 swr. It seems to work esp[ecially well on 40m for me, although I have done little on 20m where it is supposed to shine.

Anyhoo, how do you suppose that bend is affecting things, propagation pattern?  Also, does poor transmit efficiency also translate to receive performance?  For you G5RV dislike types, what would you use in its place? I have a 50' high pulley approx above my window, but that point is only about 20' from the back property line. The front leg slopes down to a small tree in the front yard. Lot is 40x100.

When daylight I can pull more readings at the antenna but since I dont run the radio out there what does that tell us?
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AC2EU
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« Reply #13 on: June 21, 2013, 08:46:42 PM »

OK I did make one error in my description, the antenna has a much sharper angle between the legs than 170 degrees, its more like (again, guess but from a better angle) 110-120 degrees, almost an expanded L shape.

As to the measurements, since I have a tuner, does it matter to how much actual power is radiated of my radios 100watts?

Also, I will take more measurements at the antenna using a 12" patch to the analyzer.

Did you guys need me to post the X R values as well as lowest SWR points and meter ohms?

The original question of course is how to make use of this analyzer and possibly make changes to my antenna/feedline based on what it reports. (and learn something in the process)

I assume that this antenna is primarily an 80M?
None the less, 12' of coax is rather short for that frequency. The coax AND the antenna are a system. There are interactions that change with frequency. Decide what band you want to optimize and use a coax calculator to get the most advantageous
match at the shack end.

All of this can get very complicated, very quickly, but it's worse when you are working with an erratic load ( G5RV).
I highly recommend the ARRL antenna book for a more in-depth study.
It's fascinating stuff.
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CHRISDX
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Posts: 244




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« Reply #14 on: June 21, 2013, 08:56:41 PM »

I'm not ready for very complicated. Just the basics. So I guess my questions I asked was about the practicalities of the antenna:

1) transmit - where do I stand on 80/40/20/10 as it is. I can get acceptable swr, but what is the efficiency. I assume that if the radio makes 100w, the antenna puts out in actuality, some fraction of 100watts. What is the radiated power likely to be with this scenario on each of those bands?  Just estimating for discuission, are we talking 50% efficiency, where I am radiating half my power to the world?

2) receive - does the same efficiency apply on recieve? would a "more efficient" antenna also receive that much better?
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