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Author Topic: Moved Dipole higher, got higher SWR.?  (Read 5305 times)
N6SBN
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« on: September 16, 2013, 07:34:32 AM »


  I moved a ZS6BKW / G5RV from 26' to 31' and my SWR rose from 1.5 to 2.  Mmmmm   Any suggestions...
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K3VAT
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« Reply #1 on: September 16, 2013, 08:03:50 AM »


  I moved a ZS6BKW / G5RV from 26' to 31' and my SWR rose from 1.5 to 2.  Mmmmm   Any suggestions...

What band?  What size G5RV? What feedline (and L)  This may be normal, depending on a number of factors, so we need more info.

73, Rich, K3VAT
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WX7G
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« Reply #2 on: September 16, 2013, 08:24:11 AM »

If the resonant frequency shifted the antenna length can be trimmed. If the minimum VSWR changed there is nothing you can do.
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PA1ZP
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« Reply #3 on: September 16, 2013, 09:03:19 AM »

Hi

Due to the higher level above the ground the influences of this ground gets les on the impedance.
The impedance got higher that way, therefor the SWr might have gotten a bit higher..

It is hard to say what can be done about this because the ZS6BKW is a multiband antenna.
If you change one band the others change to,  and might get even worse as before.

73 Jos
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KQ6Q
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« Reply #4 on: September 17, 2013, 01:35:16 AM »

it's higher, so it radiates more to the horizon. It's further from ground, so the feed point impedance changes. 2:1 SWR is acceptable. just operate. If you want to change something, add a few feet of coax to the feedline and see what changes on which bands. if not using coax, don't worry about it.
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W5DXP
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« Reply #5 on: September 17, 2013, 04:45:47 AM »

I vary the length of the 300 ohm section from 39' to 43.5' on my ZS6BKW to achieve the following SWR curves.

http://www.w5dxp.com/ZS6BKsWr.JPG
« Last Edit: September 17, 2013, 04:53:31 AM by W5DXP » Logged

73, Cecil, www.w5dxp.com
The purpose of an antenna tuner is to increase the current through the radiation resistance at the antenna to the maximum available magnitude resulting in a radiated power of I2(RRAD) from the antenna.
W2WDX
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« Reply #6 on: September 18, 2013, 03:57:59 PM »

People always change the radiator length on these antennas, which is the wrong approach. Doing this lowers efficiency. Then they grumble the G5RV antenna is poor performer. While not the most efficient, since it is after all a compromise design, given the correct circumstances and proper tuning technique they can perform well. This antenna is a stub tuned system and tuning should be accomplished (based on height) by changing the stub tuning length, not the radiator length.

I always prefer a fully tuned balanced system, long and high dipole with balanced line and balanced tuner. Lowest loss, highest efficiency multi-band.

John, W2WDX
« Last Edit: September 18, 2013, 04:00:13 PM by W2WDX » Logged

KB6HRT
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« Reply #7 on: September 19, 2013, 12:42:43 PM »

Every body has there owe thought on the G5RV and so do I, when testing antennas about 5 years ago found the G5RV to work better at this location than all the other antennas I tried it against for what I was doing. Started out with a Double Bazooka cut for 75m at 36' to the apex, A-B tested it against a 75m dipole in the same plane, DB was quiter on receive, then went to a coiled 75m dipole it received was quite but SWR was very narrow and signal were down some from the other antennas, Then compared a 43' vertical against the other 3 antennas it had a stronger incoming signals at night, but brought in more noise but did pull in signals some farther away some of the time, mostly at night on 75m. I then tried a 204' G5RV against other 3 antennas on 75m, this antenna work 75m alright but was down one S unit from a 102' G5RV or receive, both antennas were setup in the same plane for these tests as were the other antennas. Today am using a 102' G5RV using 300 ohm twin lead with the Apex at 36' with app 80'+ of RG8X feed where I found the lowest null for 3940.00 where It is used most days.  Have a Yasue VL-1000 amplifier which has a 3 to 1 antenna tuner built in an tracks 80-40-20-15-12-10m when used. Point being the 102' G5RV fills my needs for what I do using it for, are there better antennas of course, but not for my needs. In the last 5years the antenna has worked swell for me. I do use a Mini Beam primarlly when on 10-20m an it works quite well!.............kb6hrt Cheesy
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W5DXP
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« Reply #8 on: September 19, 2013, 02:57:27 PM »

I then tried a 204' G5RV against other 3 antennas on 75m, this antenna work 75m alright but was down one S unit from a 102' G5RV or receive, ...

In what direction? A 204' dipole is going to have gain and a more narrow beamwidth on 75m over a 102' dipole. I would be very surprised if a 102' dipole beat a 204' dipole broadside on 75m.
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73, Cecil, www.w5dxp.com
The purpose of an antenna tuner is to increase the current through the radiation resistance at the antenna to the maximum available magnitude resulting in a radiated power of I2(RRAD) from the antenna.
W2WDX
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Posts: 188




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« Reply #9 on: September 19, 2013, 04:21:56 PM »

Not really. It depends on height and tuning of the stub. The same stub at a different radiator length at the same height would create differing radiation efficiencies between the two antennas. At a lower height the shorter antenna may work better, since it is closer to its ideal height for its design.

G5RV is very dependent on height for a given size and stub tuning in terms of its efficiency. This is why so many people have trouble with or came to the erroneous conclusion they are not good designs. Where it lacks is in this very tweaky nature, they are unforgiving (contrary to what people say, especially those who sell them.) Like any antenna you have to give it the environment it is designed for, otherwise it will not function efficiently. And a low "SWR" says nothing about how efficiently the antenna is radiating or receiving, especially with a G5RV. As is so often the case it's not the design that is lacking, it's the application and lack of understanding by the end user.

So it makes perfect sense a shorter G5RV worked better in KB6HRT's specific installation. His situation favored the requirements of the smaller antenna.

Too many factors effect antenna performance. Ground conductivity, terrain, structures, etc. This is one of problems in relying on modeling, especially on basic versions of these programs. If you are modeling for "corn country" with flat consistent terrain, sure ... works great. However when you get into a suburban or worse yet an urban environment, you can only accurately model predictions if you can achieve enough height to overcome these unmodelable terrain variables, and that's high especially on the low bands. Where I am, the Bronx section of NYC, the topography randomly varies several hundred feet within areas as small as a mile, there is exposed bedrock cliffs, normal soil, a nearby ocean, the Hudson river, huge buildings scattered about, and all this within a wavelength on 160m. How does one model this? You can't. Not with the basic versions of modeling software.

This is why the basic approach to putting up an antenna should be "What antenna would work best in my space?" and not, "What is the best antenna made". Too many times people hear about a certain design and do not consider whether or not they have the installation required for a particular design. Ordinary resonant dipoles really only work their best at 1/2 wavelength above the ground or higher. G5RV work best at their design height or much much higher. OCF's ... well ... that's something else. Who really knows what's going on there?  Grin

John, W2WDX
« Last Edit: September 19, 2013, 05:01:35 PM by W2WDX » Logged

KB6HRT
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« Reply #10 on: September 19, 2013, 06:27:58 PM »

Thank You JOHN WZWDX,
 You said it way better than I could have!  Tried a May Pole 80-40 an 15m it was more omni direction with signals an worked rather well, both on 40 & 80m but at night brough in much more band noise, did get out well in most directions, more of a 360 deg antenna, was down on bands it was not cut for, had to use my LDG PRO
1000 tuner.   The VL-1000 tuner likes the G5RV a lot better an the VL-1000 would only tune the May Pole on 80-40 an 15m.............kb6hrt
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K1PJR
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« Reply #11 on: September 19, 2013, 08:05:21 PM »

Forget about the increase in SWR. It's irrelevant. There are numerous articles that dispel the SWR myth. Read Reflections by Walter Maxwell. It's on the web. Reflected power from the antenna is reflected back to the antenna. Any loss will occur in your feed line depending how efficient it is. It's about antenna efficiency. If the antenna is resonant just about all your power is radiated.

Resonance is the absence of reactance. That can be achieved by physical adjustment of lenght or elimination of reactance with a low loss manual tuner. Read everything you can and experiment. You would be amazed at some of the results you can achieve.   Smiley
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WB6BYU
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« Reply #12 on: September 19, 2013, 10:15:51 PM »

Quote from: W2WDX

...It depends on height and tuning of the stub. The same stub at a different radiator length at the same height would create differing radiation efficiencies between the two antennas. At a lower height the shorter antenna may work better, since it is closer to its ideal height for its design...



I don't think there is as much variation in practice as you might think.  Certainly, while
the feedpoint impedance will vary somewhat with height above ground, the resulting change
in SWR will make little difference in the antenna efficiency, or the efficiency of the feedline.
There may be an "ideal height for its design" in terms of getting a perfect match, but the
greatest variation with height will be in ground losses, which will be roughly the same as
a function of height for most types of horizontal antennas.  Really the only optimization for
height is a function of vertical angle of radiation, which has nothing to do with antenna
efficiency, and also applies equally to most horizontal antennas.  So the 204' double G5RV
will have the same angle of radiation broadside as the 102' version - there is no significant
difference between them in terms of "optimum height for its design."

There are lots of other things that also can affect the SWR for the same antenna raised up
a few feet:  the way the matching stub is dressed, proximity between the ends and buildings
or other wires, the angle in the midpoint, etc., and these will vary from one band to the
next.  So the SWR may go up on one band and down on another.

Probably the most significant difference between the 204' and 102' versions will be the
azimuth radiation patterns:  this will be slight on 80m, but much more on the higher
bands where there will be a complex pattern of lobes and nulls, such that which antenna
seems better for any particular contact will depend on the distance, direction, frequency,
and ionospheric conditions.


The behavior of the G5RV is pretty predictable in terms of impedance and radiation pattern.
You can get a little variation due to how it is installed, especially if the wires are sloped
downwards too much.  The choice of wire and stub lengths are designed to be a compromise
that gives a usably low SWR on 80, 40, 20 and 12m:  while you can often load it with a
tuner on other bands, the coax losses are likely to be high, as shown here:
http://www.vk1od.net/antenna/G5RV/index.htm

Another major source of quirky behavior is the lack of a balun at the junction between the
matching stub and the coax:  common mode currents on the coax can mess up the pattern
and, if the coax is laying on the ground, increases the losses.  (This isn't the fault of the
antenna itself, but how it is fed.)  While Lou Varney G5RV himself originally recommended
against using a balun, that was due to the poor performance of the baluns available in
the 1950s.  A good 1 : 1 current balun at that point will often make a big difference.

Because the tuning is a compromise, you'll likely need an antenna tuner on most bands,
regardless of what some manufacturers may claim.  Their insistence on a minimum of
60+ feet of coax will improve the SWR by increasing losses.  (True, G5RV recommended
this, but it was because the tube-type transceivers of the time had a limited ability to
match impedances below 50 ohms:  his recommended coax length transformed the low
impedance up to a workable value on 80m.  This isn't a limitation for modern wide-range
tuners.)
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W5DXP
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« Reply #13 on: September 20, 2013, 04:43:33 AM »

If the antenna is resonant just about all your power is radiated.

To avoid any confusion, it should probably say "antenna system" above. The purpose of an antenna tuner is to resonate an otherwise non-resonant "antenna system". A tuner does much more than simply make the transmitter happy.

http://www.w5dxp.com/OWT1.htm

For instance, a 1.25WL Extended Double Zepp antenna wire is non-resonant but when resonated by a tuner driving low-loss open-wire transmission line, can be a very efficient antenna system.
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73, Cecil, www.w5dxp.com
The purpose of an antenna tuner is to increase the current through the radiation resistance at the antenna to the maximum available magnitude resulting in a radiated power of I2(RRAD) from the antenna.
K3VAT
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Posts: 715




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« Reply #14 on: September 20, 2013, 04:55:54 AM »

If the antenna is resonant just about all your power is radiated.

To avoid any confusion, it should probably say "antenna system" above. The purpose of an antenna tuner is to resonate an otherwise non-resonant "antenna system". A tuner does much more than simply make the transmitter happy.

http://www.w5dxp.com/OWT1.htm

For instance, a 1.25WL Extended Double Zepp antenna wire is non-resonant but when resonated by a tuner driving low-loss open-wire transmission line, can be a very efficient antenna system.


... and hence the statement at the end of all of W5DXP's postings: "The maximum power transfer theorem works just as well for non-resonant antennas as it does for resonant antennas."
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