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Author Topic: 20 meter dipole and a tuner.  (Read 4727 times)
KF5VZW
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Posts: 17




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« on: October 30, 2013, 05:59:00 AM »

I have a 20 meter dipole that works great on that band.  I recently bought a LDG Z100 pro auto tuner to use with my Icom 735.  I hooked up the dipole to the tuner and it appears to work but I can't talk to anyone that is not on 20 meters.  What I try is this.  I find a conversation, move slightly off the frequency to tune. The tuner goes through the motions of tuning and the SWR light comes on like it is tuned.  I go back to the conversation and try to jump in when I can but I can't get anyone to acknowledge my transmission.  The power meter is moving on the radio indicating I am transmitting but no luck on anything but 20 meters.  I have tried both higher and lower than 20 but no luck.  I should at least be able to tune it to a shorter wavelength but no luck.

Does anyone have any ideas.  A friend is loaning me a G5RV antenna soon so I should be able to do better with that but I am still curious about the 20 meter dipole.

73s
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M6GOM
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« Reply #1 on: October 30, 2013, 06:06:40 AM »

A 20m dipole is better on the 20m band than a G5RV is. Go to www.websdr.org and try some of the SDR recievers there to see if you can hear yourself.
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WB6BYU
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« Reply #2 on: October 30, 2013, 07:26:48 AM »

How long is your feedline and what type of coax are you using?

The SWR likely is high on the coax when you aren't on 20m, so the losses will
be higher.  Depending on the type, length and band of operation, you could
loose 10dB (which is 90% of your power) in the coax due to high SWR.  With
a short, low-loss feedline you'll get more power to the antenna.

The tuner presents a low SWR to the rig so it can deliver full output power,
but doesn't affect the SWR on the coax between the tuner and the antenna.
For good efficiency with long runs of coax you still need to provide a reasonable
match at the antenna - getting it down to 5 : 1 or so will improve matters even
if you still need the tuner.
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IZ2UUF
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Posts: 63


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« Reply #3 on: October 30, 2013, 08:11:27 AM »

I have a 20 meter dipole that works great on that band.  I recently bought a LDG Z100 pro auto tuner to use with my Icom 735.  I hooked up the dipole to the tuner and it appears to work but I can't talk to anyone that is not on 20 meters.  What I try is this.  I find a conversation, move slightly off the frequency to tune. The tuner goes through the motions of tuning and the SWR light comes on like it is tuned.  I go back to the conversation and try to jump in when I can but I can't get anyone to acknowledge my transmission.  The power meter is moving on the radio indicating I am transmitting but no luck on anything but 20 meters.  I have tried both higher and lower than 20 but no luck.  I should at least be able to tune it to a shorter wavelength but no luck.

Hello.

The problem is the great mismatch loss due to your feed line.

Here some figures, calculating a 20m-band dipole fed by 30m of RG-213 and tuned:

21 MHz
Impedance at feed point: 416+j958 (SWR:51.6)
30m RG-213 loss: -7.546dB
Impedance at end of feedline: 6.241-j19.856
Tuner loss (Q=100):-0.6 dB
TOTAL LOSS: -8.146 dB

28 MHz
Impedance at feed point: 2938-j713 (SWR:62.4)
30m RG-213 loss: -8.792dB
Impedance at end of feedline: 6.058+j2.818 
Tuner loss (Q=100):-0.2 dB
TOTAL LOSS: -8.992 dB

Furthermore, this calculation is considering tuners with inductors having Q=100, which is realistic with manual tuners using big coils, but I don't know for automatic tuners, that use small toroidal inducances.

That dipole should work very well in higher bands, actually better than monobanders since its higher gain (1-3dB); up to about 1.1/1.2 wavelengths, lobes are still perfect. However, too much power is lost in the feedline due to mismatch.
Using a impedance transformer (like a 9:1) should reduce the line mismatch, but could make that dipole useless on 20m.
Other solutions:
  • tune at antenna feedpoint (remote tuner)
  • use a 450-ohm window line instead of RG-213: this would drop losses quite a lot (e.g. in 15m from -7.546dB  to -0.361dB);
  • run a 450-ohm window line till somewhere you can hide a normal automatic tuner (like in the attic); then continue with regular RG-213 of any length.

73 de Davide IZ2UUF
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Davide IZ2UUF - FISTS #16285 - SKCC #9531 - JN45nk
K0ZN
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Posts: 1553




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« Reply #4 on: October 30, 2013, 09:57:21 AM »

Hi.

I gathered from your comments that you are trying to use that 20 meter dipole fed with coax on other bands.  Short answer:  Bad idea. Don't.  

Reasons: A tuner is NOT a "magic black box" that will make any antenna work on any band. Just because you can hear does NOT mean you can transmit. Your tuner may find a "match" but that does not guarantee all is well. For example, if you try to load a 20 M dipole on 40 M the feedpoint impedance is very low....a few ohms...and you probably have (I am not going to take time to calculate it) an SWR of 25 : 1, further the antenna is way too short and inefficient AND you are probably loading up and radiating from the shield of the coax and the losses in the coax will be quite high.  ALL of those are bad situations. On 10 M the feedpoint impedance is probably around 2,500 ohms and you are feeding the antenna with 50 ohm coax....again, the SWR will "astronomical" and the previous problems all exist. You might get it to work
"fairly well" on 17 M, but that would be about it.  You can use a dipole for multi band work, but you must feed it with low loss line such as ladderline or open wire line, use a balanced tuner or balun between your current tuner and the balanced line.

Very respectfully:  You would be well served to spend a little time studying the ARRL Antenna Book to pick up some basic knowledge of antennas and transmission lines.
Basic Antenna theory is not complicated. Antennas are not difficult, but they are also not forgiving of assumptions and improper feeding, etc. The time and effort you spend picking up some knowledge will be well rewarded with fewer problems (possibly even avoiding equipment damage!) a better signal and more fun on the air. The cold fact is that this is a technical hobby and the more you know, the better your results will be.

Again, the foregoing comments are intended to help you enjoy the hobby, not criticize. The old saying that knowledge is power is very true in Amateur Radio.

73 & Good luck.   K0ZN
« Last Edit: October 30, 2013, 10:01:06 AM by K0ZN » Logged
IZ2UUF
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« Reply #5 on: October 30, 2013, 10:27:54 AM »

Reasons: A tuner is NOT a "magic black box" that will make any antenna work on any band. Just because you can hear does NOT mean you can transmit. Your tuner may find a "match" but that does not guarantee all is well. For example, if you try to load a 20 M dipole on 40 M the feedpoint impedance is very low....a few ohms...and you probably have (I am not going to take time to calculate it) an SWR of 25 : 1, further the antenna is way too short and inefficient AND you are probably loading up and radiating from the shield of the coax and the losses in the coax will be quite high.  ALL of those are bad situations. On 10 M the feedpoint impedance is probably around 2,500 ohms and you are feeding the antenna with 50 ohm coax....again, the SWR will "astronomical" and the previous problems all exist. You might get it to work
"fairly well" on 17 M, but that would be about it.  You can use a dipole for multi band work, but you must feed it with low loss line such as ladderline or open wire line, use a balanced tuner or balun between your current tuner and the balanced line.

Very respectfully:  You would be well served to spend a little time studying the ARRL Antenna Book to pick up some basic knowledge of antennas and transmission lines.

Sorry K0ZN, I don't agree on some of your comments.

First, there is no reason for the line to radiate in presence of any mismatch; the line might radiate if, for example, the balun is designed to work on one band and it performs badly on the band we are tuning our aerial. But in this case, radiation is not due to mismatch, it is due to a wrong balun.
These concepts are widely explained by W2DU in his book, "Reflections".
The efficiency problem is true when feeding short antennas, like using a 20m dipole on 80m; the low R makes radiation efficiency very low.
Instead, when feeding long antennas, the higher R increases radiation efficiency.

Also... a tuner doesn't try to find a match, the tuner gives an exact match, or if you are out of its tuning range, you won't be able to tune it at all.
The problem is only one: the great loss in low-impedance feedlines, so high that can kill any transmission.
Receivers are so sensible that they can withstand a 10dB loss without a glitch; for example, beverage antennas are very very lossy, but they work for reception because receivers are very sensible and beverages have good S/N ratio. But in TX, 10 dB lost means wasting 90% of the TX power in heat.

For example, feeding a longer dipole with the right feedline, gives much better results than an usual half wave dipole. For example, a 13m long center-fed doublet dipole fet by 30m of 450-open window line, gains from 1 to 3 dB more than every single-band 1/2 wave dipole in all frequencies from 14 to 30 MHz, though it doesn't resonate on any of them.
And this, of course, carefully accounting for line and tuner losses.
In other words, a 1/2 wave dipole is a compromise antenna for those who accept a lower performing antenna and single band operation for the commodity of using RG-213 and avoiding a tuner.

In conclusion, tuning a 14 MHz dipole on higher bands can be a good idea only if the line mismatch loss problem is faced and solved.

73 de Davide IZ2UUF
« Last Edit: October 30, 2013, 10:35:58 AM by IZ2UUF » Logged

Davide IZ2UUF - FISTS #16285 - SKCC #9531 - JN45nk
W5WSS
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« Reply #6 on: October 30, 2013, 05:47:24 PM »

Iz2uuf I agree 100%.
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KF7ITG
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Posts: 82




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« Reply #7 on: October 30, 2013, 06:56:56 PM »

Perhaps this link may be of some help. Some general 411.

http://www.hamuniverse.com/n4jaantennabook.html

73
James
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AE5X
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« Reply #8 on: October 30, 2013, 07:10:02 PM »

Bill, let me make a recommendation and short-cut a lot of the reading that's been suggested for you:

Thank your friend for the kind offer of a loaned G5RV, then politley decline. Instead, replace the coax feeding your current 20m dipole with 450-ohm ladder line. Then use your tuner and enjoy an antenna with greater efficiency than a G5RV and more versatility than your current configuration (with the tuner it will operate 10-20m).

My experiences with an 80m version of a ladder-line fed dipole are 300+ countries and 8B-DXCC - all with one dipole.
http://www.ae5x.com/blog/2011/02/07/ladder-line-fed-dipole-and-g5rv/

John AE5X
« Last Edit: October 30, 2013, 07:14:12 PM by AE5X » Logged
KJ7WC
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Posts: 69




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« Reply #9 on: October 30, 2013, 07:25:05 PM »

Here's the thing that I've yet to comprehend about a G5RV. The antenna is intentionally non-resonant so that most tuners can handle the mismatch on any amateur band. Okay.

No matter what transmission line you use, a variable frequency will cause a standing wave on the feed line. A balanced conductor is certainly more efficient in the presence of a standing wave. However, I must argue that the length of coax fed to the balun on a G5RV matters just as much as the length of window line up to the antenna. You see, if there is a mismatch on the balanced conductor, there will be a mismatch on the coax. Whenever a mismatch is present on a transmission line, a transformation will occur that is wholly dependent on the length of that transmission line in wavelengths.

I will echo John's sentiment - decline the offer for a G5RV. Antennas are most useful in whole multiples of a half-wave. I've got a half-wave 40m dublet (66' of wire) inverted-V in my yard fed with window line. My rig can operate 40-20-15-10m without a tuner. Why? I have sections of window line to change the length of the feed, and consequently present a 50-ohm load at the balun. The antenna works well, because it is resonant on four bands. The next step is to add a double-L network with a pair of roller inductors. The reason I don't use the antenna on 80m or 160m is because it is physically less than one-half wavelength.

Glad IZ2UUF mentioned "Reflections." I still need to purchase the book. You should get one, and an antenna book.
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KF7ITG
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Posts: 82




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« Reply #10 on: October 30, 2013, 07:47:38 PM »

This is a great book to add to your antenna library. Practical Antenna Handbook in PDF. Actually I like the paperback version from Amazon for around $12. Although the PDF version is great on your IPad if you have a PDF reader app installed.

http://www.apparentlyapparel.com/uploads/5/3/5/6/5356442/_____practical_antenna_handbook_fourth_edition_carr.pdf
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WB6BYU
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« Reply #11 on: October 30, 2013, 08:58:15 PM »

Quote from: KJ7WC

Here's the thing that I've yet to comprehend about a G5RV. The antenna is intentionally non-resonant so that most tuners can handle the mismatch on any amateur band.



Well, not exactly.  Actually it is designed to be 3/2 wavelengths on 20m, a resonant length
that gives a 6-lobed pattern that was well suited for a lot of DX directions from G5RV's location.
The half wavelength feedline, while operating at a moderate SWR, repeats the ~100 ohm impedance
of the antenna feedpoint at the bottom where it connects to coax.  This gives a relatively low SWR
on the coax (especially when using 75 ohm line, or the 72 ohm twinlead that was more common back
in the 1950's than it is now.)

That particular combination also presents a usable SWR on 40m, the bottom of 80m, and on 12m,
so coax losses are relatively low on those bands.  On other bands the SWR on the coax is higher,
so losses will increase (even if your tuner can match the impedance.)




Quote

...I must argue that the length of coax fed to the balun on a G5RV matters just as much as the length of window line up to the antenna...



This is true with regards to the impedance seen at the tuner end of the coax, but the length of the
window line determines the actual SWR on the coax, and that affects the range of impedances that
can appear at the tuner.  For example, finding a combination that gives an SWR of 2 : 1 on the coax
limits the impedance the tuner has to match to between 25 and 100 ohms (likely with some reactance),
consistent with the SWR of 2 : 1.  If a different length of window line, or operation on a different band,
causes an SWR of 10 : 1, then the corresponding limits are 5 ohms to 500 ohms instead.  The former is
within the matching range of many built-in autotuners, while the second may not be.




Quote

You see, if there is a mismatch on the balanced conductor, there will be a mismatch on the coax.



This is NOT true, and it is an important factor in how many antenna matching systems work.

For example, imagine a dipole with an impedance of 50 ohms fed through an electrical half
wavelength of 600 ohm open wire line.  The SWR on the open wire line is 12 : 1, but because
it is a half wavelength long the impedance at the bottom end is the same as at the top,
namely 50 ohms.  So there is a high SWR on the open wire line, but 1 : 1 on the coax
connected to it.

In fact, that's how you operate your antenna system without needing a tuner:  how do you
think you are presenting a 50 ohm impedance to the balun on each band by changing the
length of the feedline if the window line is not operating at a high SWR?  If it shows 50 ohms
to the balun, then the SWR on that line must be 450 / 50 = 9 : 1.


To get back to the OP, there are several types of antennas that will allow you to operate
multiple bands.  One is to add dipoles for different bands to feedpoint of your current dipole:
I often use 80m, 40m and 20m dipoles strung out in different directions from the same
feedpoint (and the 40m dipole works well enough on 15m).  The "doublet" that several
others have mentioned is simply a dipole of non-critical length fed with ladder line to a
tuner in the shack:  the ladder line is capable of operation at high SWR with lower loss
than coax, making it less lossy for multiband operation.



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KF5TBS
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« Reply #12 on: October 30, 2013, 09:15:17 PM »

Just a note on my experience - I tied my 20 meter dipole to a hommade 1:1 balun and fed the tuner with LMR400 coax.  I was able to get 20 meters best, but still able to get 15 and 10 meter stations (when the band is good).
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K0ZN
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« Reply #13 on: October 30, 2013, 09:45:25 PM »

Hi, Davide.....

Well....it seems every time I try to keep things simple for a new ham, I get my post "over analyzed". My thought being that not every ham wants a detailed engineering analysis of the issue.....but maybe I am wrong.....   Generally, I agree with you and fully appreciate and understand what you say. However, there is some body of experience to show that if you feed a highly mismatched BALANCED radiator directly with UN-balanced coax, you WILL get some RF on the shield. Respectfully, I am not going to participate in a technical debate here. My intended point, which I STILL take, was to make the point that feeding a single band dipole on multiple bands WITH COAX is a BAD practice.....tuner or whatever......

Believe me, I fully understand the multiband center fed system concept and operation: that is precisely what my main HF antenna is a 128 ft.(~39 m) dipole (or Center fed Zepp/doublet or whatever you want to call it) fed with 450 ohm line to a Johnson KW Matchbox tuner. I use it on all bands with excellent results. Arguably, this type of antenna, fed with low loss line, is the best multiband HF antenna out there.

73,  CUL,   K0ZN
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GW3OQK
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« Reply #14 on: October 31, 2013, 02:34:47 AM »

I believe the same as you K0ZN and use the same sort of dipole/doublet (36m) and 450 ohm line. NO problems and great results on all bands.
73
Andrew
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