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Author Topic: How to obtain maximum usable bandwidth on 160M Inverted L?  (Read 1936 times)
KH6AQ
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« Reply #15 on: September 05, 2017, 10:24:47 PM »

The OPs requirement is 60 kHz SWR bandwidth.
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WA7PRC
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Posts: 1774


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« Reply #16 on: September 06, 2017, 01:32:50 AM »

K4SX describes his cage inverted L here: http://www.smeter.net/antennas/160-meter-cage-inverted-l.php He says his 2:1 VSWR bandwidth is 200 kHz.

My experience is similar on 80m using a cage inverted vee... 500 kHz VSWR bandwidth:
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WB6BYU
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Posts: 16798




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« Reply #17 on: September 06, 2017, 08:48:52 AM »

Quote from: N1LO

Another possibility might be to use stagger tuned extra wires- that is one cut slightly longer than the other and fanned apart a bit. This could work for the radials as well as the radiator




I used this approach successfully for a dipole that covered the whole 80m band at less than 1.5 : 1 SWR,
but it wasn't as easy as it might appear.

I used 7- or 10-conductor ribbon cable, initially cutting each side to staggered lengths as calculated from
the standard dipole formula.  When I put it up the high end SWR bandwidth was about 3.5 MHz, so
I had to shorten the centers.  I tied a knot about 2/3 of the way out each element and separated the
wires from the ends to that point, then grouped them into two bundles with the even numbered wires in
one and the odds in the other.  By spreading those two bundles apart I was able to get the wide-band
performance I was looking for.  I can adjust the end frequencies of the SWR passband by changing the
angle between the two bundles on each end.

That was really handy for an 80m antenna on Field Day, until someone took it down with his pocket knife
at the wrong point.


But, as others have pointed out, even with a wide-band antenna you'll still have to retune your amp as
you move across the band.
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ONAIR
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Posts: 3482




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« Reply #18 on: September 06, 2017, 11:19:40 AM »

To get wider bandwidth you basically need a 'thicker' antenna. For a wire antenna this means running several wires in parallel with some space between them. Radials are part of the antenna too, so they may need to be broadbanded: run wires in parallel as for the antenna, or put nearer ground so that ground losses reduce Q (but this will reduce efficiency).
    How about just wrapping PVC poles with aluminum foil tape?
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WB6BYU
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Posts: 16798




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« Reply #19 on: September 06, 2017, 12:21:59 PM »

Quote from: ONAIR

How about just wrapping PVC poles with aluminum foil tape?




You really need a couple feet of diameter, much larger than any PVC pipe I'd want to erect.

For an 80m "T" antenna I used two vertical wires spaced about 12' apart for most of the height.
That still didn't cover the whole band, but was a noticeable improvement over a single wire.
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KE2TR
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Posts: 569




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« Reply #20 on: September 11, 2017, 05:24:56 PM »

A few years back I used a coaxial inverted L on 160m when one of my old qth's didn't have allot of room, used two gull wing elevated radials that I stapled against some wooden stockaded fence and was able to get over 100khz bandwidth under 2:1. Years later I installed the same type antenna for 75m and was able to get close to 300Khz bandwidth under 2:1. Basic antenna was like a half of a duoble bazzoka but it was all made of higher grade RG59U, no twinlead.
PS it worked good enough for local rag chews but also excellent for DX which really surprised me for such a simple antenna.
« Last Edit: September 11, 2017, 05:27:02 PM by KE2TR » Logged
N3DT
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Posts: 1220




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« Reply #21 on: September 11, 2017, 06:42:19 PM »

KL7CW's suggestion about the QST article years ago on broadbanding the 80M dipole may work. What it involves is adding a length of RG11 or RG6 to the shack end of your antenna system to your transceiver. What I did was I had the SWR trace from my AA600 analyzer at the shack end and in the RigExpert software you can add lengths of various types of coax and I found that if I added 50' of RG11, my 80M dipole ended up being well below 2:1 across the band. It really works too. Not sure how to figure that for 160M, but I'm sure it could be done, but also not sure how much broadbanding it would accomplish on 160, but it might be worth a look at that QST article on 'Broadbanding the 80M dipole". As I remember the author described the process to go through to figure what you need, at least on 80M. I'm sure it could be reconfigured for 160M. It also depends on how long your feed line is as to how much 75Ω line to add.

I discovered it quite by accident while playing around with my RE software trying to shift the resonance point on my 80M dipole, but I do remember reading about the procedure. The QST article is mentioned in my thread about "Broadbanding my 80M antenna" if that would be an easier way to find the article. Worked great on my 80M dipole at 65' and the AL80B loves it across the band. 50' of RG11 on 80M is negligible for the benefit derived and even 100' on 160M, the same.
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W9IQ
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Posts: 1561




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« Reply #22 on: September 11, 2017, 07:31:47 PM »

Quote
KL7CW's suggestion about the QST article years ago on broadbanding the 80M dipole may work. What it involves is adding a length of RG11 or RG6 to the shack end of your antenna system to your transceiver.

Adding a length of coax to the feedline of the antenna will potentially do any combination of three things:

  •      1.) Shift the impedance due to the impedance transformation of the additional length of the coax
         2.) Shift the resonant point of the antenna system due to common mode currents and the additional length of coax that is acting as part of the antenna.
         3.) Lower the SWR in general due to the additional losses of the added feedline

In the first case, the SWR dip will be shifted but the SWR bandwidth will not change.

In the second case, the resonant point of the antenna may shift. The efficiency of the antenna may go up which would reduce the SWR bandwidth or the efficiency may go down, which would increase the SWR bandwidth. Given the unbalanced nature of an inverted L antenna, this is a likely case.

In the third case, the  SWR will improve but it is at the expense of power lost due to the additional feedline. The antenna is not performing any better than without the added length of feedline but the operator feels better due to wider SWR bandwidth at the transmitter. This effect would be minimized at 80 and 160 meters due to the very low loss of the feedline at these frequencies.

- Glenn W9IQ
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- Glenn W9IQ

I never make a mistake. I thought I did once but I was wrong.
N3DT
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Posts: 1220




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« Reply #23 on: September 12, 2017, 07:01:05 PM »

I missed that part about the OP using an inverted L. I have no clue how adding RG11 to an inverted L would change things, but if the OP has access to one of the RigExpert analyzers, it can easily be  estimated in the analyzer's software without even doing anything physically. The QST article was on a dipole. Wonderful devices, these new analyzers.
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W9IQ
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Posts: 1561




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« Reply #24 on: September 12, 2017, 08:21:47 PM »

David,

Just to be clear, that does not change my analysis. The changing of the length of the coax still only has the three effects that I detailed - even with a 1/2 wave dipole.

- Glenn W9IQ
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- Glenn W9IQ

I never make a mistake. I thought I did once but I was wrong.
WB6BYU
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Posts: 16798




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« Reply #25 on: September 12, 2017, 08:51:14 PM »

I remember that particular article on coax matching.  It used a length
of 50 ohm coax (a multiple of 1/2 wavelength IIRC) followed by a
length of 75 ohm coax (maybe 1/4 wave or so?). It was the combination
of the impedance transformations with frequency in the two different
impedances that gave the wider bandwidth.

So it wasn't just a matter of making the feedline longer, but creating
a hybrid feedline that counteracted some of the impedance change
with frequency.  While I don't remember the details, the principle
was well described, and I have every reason to think it would work
as described.
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K4SAV
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Posts: 2346




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« Reply #26 on: September 13, 2017, 08:27:38 AM »

I remember seeing the QST article for broadbanding an 80 meter dipole using transmission lines, but I don't remember the details.  Might be worth a search to find it. I played with a model of it when I first saw the article and even still have a model of it in my EZNEC folder. As I remember, the degree of matching possible depends on antenna height.  The model I have was for an inverted vee with apex at 90 ft.  The SWR was under 1.85:1 from 3.5 to 4 MHz.  It was under 1.6 from 3.55 to 3.9 MHz.  It used 86.5 ft of RG-213 and 42.2 ft of RG-11.  The transmission line loss was 1 dB.  That's about a half dB more loss than the loss in the same length of RG-213 terminated in 50 ohms.  So that's not a very big penalty due to line loss.

The antenna itself, without the transmission lines was resonant at 3.7 MHz.

If you want to try this and don't have the procedure for determining lengths, you can always model it at whatever height you have and juggle the line lengths until you get a good SWR curve.

Jerry, K4SAV
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N3HEE
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Posts: 394


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« Reply #27 on: September 13, 2017, 03:37:16 PM »

The 2.0:1 SWR bandwidth on my inverted L is nearly 60 KHZ.  1.800 - 1.857.  It's 70 feet vertical and 80 feet horizontal.  Sits on top of a K2AV folded counterpoise.  The FCP needs an isolation transformer but that's it.  No matching devices required.  Very simple and compact 160 meter antenna for a small lot.  Works great too ! 
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Joe
N3HEE
CW Academy Advisor (Level II)
AC6LA
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Posts: 123




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« Reply #28 on: September 13, 2017, 06:08:28 PM »

I remember seeing the QST article for broadbanding an 80 meter dipole using transmission lines ... As I remember, the degree of matching possible depends on antenna height.

That technique for broadbanding an 80m dipole happens to be one of the AutoEZ illustrated examples.  And Jerry is correct about the height, as G3TXQ pointed out in the example.

See the 4th bullet on this page:  http://ac6la.com/aecollection5.html

Dan, AC6LA
http://ac6la.com
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N3DT
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Posts: 1220




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« Reply #29 on: September 14, 2017, 06:47:08 AM »

http://www.arrl.org/files/file/Antenna%20Book%20Supplemental%20Files/23rd%20Edition/A%20Simple%20Broadband%20Dipole%20for%2080%20Meters%20-%20AI1H%20-%20QST%20Sep%201993.pdf

http://www.robkalmeijer.nl/techniek/electronica/radiotechniek/hambladen/qst/1993/09/page27/
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