160M Inverted L Matching Question

[k3msb]

Stu

Yes, but if you have just a slight amount of inductive reactance (see chart below) the amount of capacitance to compensate is huge!

[k3msb]

Hi Roger

It's 69F today!  That's won't last through tomorrow though!

OK on the 320 pF.   I'm only reporting what the analyzer tells me - my antenna has +77j @ 1.820 !!

Your 320 pF equates to about -273j.    Your antenna is about 160 ft which would mean more inductive reactance than my 138 foot antenna.

I think the moral of the story is if you're going to make your antenna long, don't make it too short (if you just want to use series C) !

[AB2EZ]

Mark

If you have a small amount of series inductive reactance, why would you want to bother cancelling it?

Stu

Stu

Yes, but if you have just a slight amount of inductive reactance (see chart below) the amount of capacitance to compensate is huge!

[k3msb]

Stu

You wouldn't need to (and thank you for pointing out the obvious while I was wrapped up in the math.....  )

One needs to compensate for a moderate or large L with a reasonable C,  but you don't need to compensate a small L with an unreasonably large C !

Thanks for your help Stu -- it was good to work through this!!!

[WQ9E]

Mark,

Enjoy the warmth while it lasts because it is miserable here.

The main reason for cutting the L a little long is to allow easy matching over a fairly wide frequency range with a simple series capacitor.  For experimentation use can use a standard 2 to 4 section receiver variable with the sections in parallel for more capacitance, once you know for sure what you need you can switch to proper high power capable capacitors. 

Be careful with doorknob caps since a lot of them won't handle much current and will heat rapidly.  The little red Ceramite variety are the worst I have seen.

[k7iou]

For those of you that may want this from an old ARRL antenna handbook. I took a picture of it for a future build.
73 de k7iou

[W2PFY]

That is a very peaceful looking picture Roger. They say a picture is worth a 1000 words and I am up to 1001

I didn't read as closely as I should have but my question is, do you need an earth ground at the point where the matching variable cap is located or are above ground radials sufficient? My guess is it should be grounded for lighting protection?

[WQ9E]

That is a very peaceful looking picture Roger. They say a picture is worth a 1000 words and I am up to 1001

I didn't read as closely as I should have but my question is, do you need an earth ground at the point where the matching variable cap is located or are above ground radials sufficient? My guess is it should be grounded for lighting protection?

Thanks Terry!   I really love the quiet rural setting.  I lived in an apartment during my Ph.D. program and decided I never want to live that close to people again

In my setup a good earth ground is a requirement to protect the 18HT vertical and its concrete base but in my opinion it should be there for safety with any setup.  And as I told Mark in an email exchange you also want an RF choke across the feedpoint to bleed static discharge to ground or a high value resistor could also be used.   For RF performance elevated radials appear to be a great option.   The ground radials were an easy installation for me.  I staked them out at the start of winter and the next spring I mowed that area with the tractor deck raised fairly high for the first two months and since them I have mowed without concern.  I used some flexible copper tubing to form a wagon wheel area next to the tower and bonded that to the ground rods and the start of the radials and this is the only part I buried.

[k3msb]

Gentlemen:

Thanks for all your comments.

I shortened my Inverted-L down to about ¼ wavelength and removed the matching capacitor for the reasons indicated in previous posts.

I'm now showing a feed point impedance of about 40 to 42 ohms R and almost no reactance (it was in the single digits).   Optimal resistance for an Inverted L is 36 ohms, so I assume that my ground return isn't as good as it should be.    I'm using a 4 foot ground rod and 7 radials of about 100+ feet.

Results were pretty good during the week before Thanksgiving and during the CQ WW CW Contest as I now have 42 countries worked on 160 (quite a few in Europe and also Hawaii).

This has been a good learning experience for me and I now know why people tend to use 165 feet for their antenna length.    My Inverted L was put up two weeks before a big contest and before the onset of cold weather (so what's new…..)  and I'm already planning on how to redo it next year.

From my very brief foray onto 160M I've also gained an appreciation as to why a lot of low band enthusiasts use separate RX antennas.   I do not have the room for a permanent beverage, but a rotatable pennant is a very distinct possibility.   

I appreciate all the help you guys have provided!  Thank you very much!

[KB2WIG]

" I do not have the room for a permanent beverage, but a rotatable pennant is a very distinct possibility.   "

I've been intrigued with the "Beverage on ground" idea. ~140 feet, on the ground. Maybe crappy 75 ohm 'cable tv' cable would blend in nicely? ......  w0BTU has a nice site.

http://www.w0btu.com/Beverage_antennas.html#Beverage_On_the_Ground

KLC

[W0BTU]

I shortened my Inverted-L down to about ¼ wavelength ... I'm now showing a feed point impedance of about 40 to 42 ohms R and almost no reactance (it was in the single digits).   Optimal resistance for an Inverted L is 36 ohms, so I assume that my ground return isn't as good as it should be.    I'm using a 4 foot ground rod and 7 radials of about 100+ feet.

The theoretical Z of a 1/4w vertical is about 36 ohms. But when we bend it over at 50' and make an inverted-L out of it, the Z goes way down. I didn't model it, but I'll bet AT LEAST 20 ohms of what you measured is your ground system.

That's why I posted this link above. It helps show the advantages of making an inverted-L a little longer, one of which is significantly raising the feedpoint Z. Mine is 155' total length.
http://www.w0btu.com/160_meters.html#inv-l_antenna

EDIT: I just modeled yours, with elevated radials (I used my EZNEC model at http://www.w0btu.com/files/antenna/160m_inv-L/ and just changed the L portion). Results:
Impedance = 21.5 - J 60.64 ohms @1.800 MHz.

[R. Fry SWL]

Per NEC4.2 modeling, an inverted L with a 50-ft vertical section and an 87-ft horizontal section of 10AWG copper conductors, driven at the base of the vertical conductor shows the real term of the feedpoint Z to be about 16.5 ohms at 1.9 MHz.  Almost all of that 16.5 ohms is produced by the radiation resistance of this configuration at this frequency.
______

As an example of an average installation, using 1.9 MHz...

This NEC model driven against a set of sixteen 10AWG copper radial wires each 50-ft long and buried 4 inches in 5 mS/m d.c. 13 earth shows a feedpoint Z of about 27.6 +j 58 ohms.  The system can be resonated with a 1446 pF capacitor in series at the feedpoint.

At resonance, this system has a radiation efficiency of about 60% when the feedpoint Z is matched to the Z of the transmission line connected there. Its 2:1 SWR bandwidth is about 80 kHz.

Maximum total gain occurs at 30 degrees elevation in the direction opposite from the azimuth direction of the horizontal antenna conductor, and is 0 dBi.  System gain at that elevation angle in the opposite azimuth direction is -1.6 dBi.

R. Fry

[flintstone mop]

That is a very peaceful looking picture Roger. They say a picture is worth a 1000 words and I am up to 1001

I didn't read as closely as I should have but my question is, do you need an earth ground at the point where the matching variable cap is located or are above ground radials sufficient? My guess is it should be grounded for lighting protection?

Thanks Terry!   I really love the quiet rural setting.  I lived in an apartment during my Ph.D. program and decided I never want to live that close to people again

In my setup a good earth ground is a requirement to protect the 18HT vertical and its concrete base but in my opinion it should be there for safety with any setup.  And as I told Mark in an email exchange you also want an RF choke across the feedpoint to bleed static discharge to ground or a high value resistor could also be used.   For RF performance elevated radials appear to be a great option.   The ground radials were an easy installation for me.  I staked them out at the start of winter and the next spring I mowed that area with the tractor deck raised fairly high for the first two months and since them I have mowed without concern.  I used some flexible copper tubing to form a wagon wheel area next to the tower and bonded that to the ground rods and the start of the radials and this is the only part I buried.

I agree that for lightning protection you need a ground rod. And ground radials are the easiest to deal with. Something near 30-60 radials as long as you can afford is best for the RF return. Raised radials are a real hassle for 160M. They should be 15-20 feet high from the ground. And then they need to be tuned too. The best thing about raised radials is that 6 is all you would need.
I plan on converting my 240 dipole to an "L" utilizing one side of the 120 feet. Disconnect the present feed point. And bring down the one end and connect the vertical wire on the utility pole side of the dipole. The other side of the dipole is presently being supported in a swaying pine tree. I would have 65 feet vertical and the 120 feet horizontal. It would be a 'little long'.
A very interesting thread.
Fred