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Author Topic: Inverted L tuning question  (Read 1629 times)

Posts: 8

« on: August 01, 2008, 07:40:19 PM »

Hi all. I have put up an inverted L of approximately 150 feet in length. 60 feet of that is vertical. When I measure the SWR at the bottom of the antenns I get higher readings than I do in my shack at the end of approx 75 feet of RG8X coax. I'm assuming that the readings I get at the bottom of the antenna are the correct ones but can anyone tell me why the readings are better in the shack?

Also, I'm assuming that to cut the wire to be resident on 160 it works just like a dipole in that respect (shorten the antenna if the high end of the band has a higher SWR than the lower end of the band and vice versa).  

Thanks in advance for any replies.

Posts: 1790

« Reply #1 on: August 01, 2008, 07:57:49 PM »


RESONANCE will show up on that type of antenna as ODD MULTIPLES of 1/4 wave length on any HF band. There is a formula to determine the length for that: L = 234/F  Where: L is the desired length of ONE quarter wave and F is the Frequency in Mhz. Using that formula (and rearranging it to solve), your 150 ft. wire is (approximately) resonant at about 1.506 Mhz... i.e. it is RESONANT in the AM broadcast band; it would be a very BAD match at that length to coax without a tuner; the SWR would be sky high. You can either shorten it to about 126 ft. or lengthen it to about 378 ft. (which is 3/4 wavelength.) Half waves or multiples of a Half Wave in that situation will have extremely high Impedance, but still be RESONANT. There is a big difference between what is "Resonant" and what can be fed with 50 ohm coax and show a low SWR on the transmission line.

Respectfully, you really need to pick up a copy of the ARRL Antenna Book and do a little studying. This stuff is simple and easily understood and will help you tremendously with your antenna construction and a BETTER signal and less frustration. Antennas are way too complex for a few paragraphs on the internet. You can pick up old copies of the ARRL Antenna Book on Ebay for almost nothing. An EXCELLENT investment.

73,  K0ZN


Posts: 23

« Reply #2 on: August 01, 2008, 10:08:39 PM »

The readings are better in the shack because of the signal loss in the coax.

An SWR above 1 means part of the signal going forward to the antenna is being reflected back to the source.  But, that refelected signal back at the source is less than at the antenna due to the loss in the coax.

It turns out that using a longer piece of coax, or a coax with more loss will make the swr measurement look even better.  But your right, the truest measurement is at the antenna.

Harvey WE9L

Posts: 1054

« Reply #3 on: August 02, 2008, 05:07:13 AM »

"I'm assuming that to cut the wire to be resident on 160 it works just like a dipole in that respect" Yes. If you were to find your SWR is lowest at 1.55 MHz, for example, cutting it shorter will raise the frequency. The question is, how much?

Multiply the current length by the frequency of lowest SWR; e.g. 150' × 1.55 MHz, and this gives you a quarter-wavelength factor of 232.5. Now, divide this factor by the desired frequency, e.g. 1.8 MHz (232.5 ÷ 1.Cool to find the new length of wire (129.2'). Since the new length is 20.8 feet shorter than 150', cutting that amount from your current set-up should make the antenna resonant very close to 1.8 MHz.

A noise bridge or antenna analyzer would be handy for finding out-of-band resonances.

GL, Stew

Posts: 2440

« Reply #4 on: August 02, 2008, 06:52:06 AM »

First, the answer to both your questions is yes.

It sounds to me like you never intended this antenna to be resonant on 160.  Do you intend to add a matching network?  The length you have should be resonant somewhere around 1.25 MHz if you used bare wire.  A series capacitor of about 130 pf will tune this to about 1.83 MHz but the feedpoint impedance will go to about 123 ohms (with a good radial system).  The antenna is too long.  To get something that will tune to 50 ohms on 1.82 MHz, you will need a top section of about 120 feet and a 320 pf series capacitor.

Another option is to make the antenna a quarter wavelength long on 1.82 MHz.  That will require a top length of about 83 feet.  That will give a feedpoint impedance of about 29 ohms with a good radial system.  If you have a really poor radial system this might even match 50 ohms but the antenna gain will be poor.  So now you need a matching network to match the 29 ohms to 50 ohms (unless you want to live with a minimum SWR of 1.75). There are an infinite number of solutions for a matching network, but the one component solution is to place a 4.2 uH inductor across the feedpoint.  When doing this you will have to shorten the top wire to about 77 feet.

The matching system is your choice.  With the first approach, to change the resonant frequency all you have to do is change the cap value.  With the second approach you have to change the top wire length to change the resonant frequency.

Jerry, K4SAV

Posts: 2440

« Reply #5 on: August 02, 2008, 07:08:25 AM »

Sorry, I misunderstood.  I read the top wire as being 150 feet.  You said the TOTAL wire length was 150 feet.  My mistake.

The information I gave was good, but doesn't exactly fit the antenna you have as a starting point.  Your antenna should be resonant somewhere around 1.73 MHz.  You can use the either of the options I listed the first time to match this.  Both will require a change in the top wire length.

Jerry, K4SAV

Posts: 1214


« Reply #6 on: August 02, 2008, 08:12:38 AM »

If you have a mismatch at the feedpoint, and it sound like you do, the swr will change as you go down the coax toward the transmitter end.  There's not enough loss on 160 with a 75-foot piece of coax to make a difference in the swr as you describe.  If your antenna is 150 feet long, you obviously have inductive reactance at the need to cancel that out with a variable capacitor.

Posts: 4311


« Reply #7 on: August 02, 2008, 08:41:17 AM »

If you want to radiate even less power with an even better SWR at the shack, switch to RG-174 coax. :-)

If you want to radiate more power but suffer a higher SWR at the shack, switch to RG-213.

The lower the losses in the coax, the closer will be the SWRs at the antenna and at the shack.
73, Cecil,

73, Cecil,

Posts: 670

« Reply #8 on: August 03, 2008, 03:32:10 PM »

I would measure at the shack. The coax can act as a transformer and change the aparent SWR. Your presence at the antenna when you measure can also change the SWR. Your goal is to get the SWR at the shack reasonable so that you can radiate effectivly. At 160m coax loss will be negligable at any reasonable SWR.
  An inverted L is a vertical antenna so you need radials and they will also change the SWR at the shack.  If you use raised radials 2 resonant would be the minimum, 3 or 4 would be better, and with 6 you have reached the point of diminishing returns for efficiency.. With ground radials you need 10 or 12 minimum for resonable efficiency but they do not have to be resonant and can be zig zagged to keep in your property.   73 WD8PTB

Posts: 6642

« Reply #9 on: August 04, 2008, 12:43:58 PM »

It sounds like you have an inverted L with 60 vertical feet and 90 horizontal feet, fed at the bottom with RG8X and working against a ground rod of some kind.  That is not resonant on 160 meters.  And, the feed point will not be 50 ohms impedance.
You are using RG8X with a characteristic impedance of 50 ohms... that means if terminated in 50 ohms the impedance will be the same at all points alaong the coax line.  Since you are not terminated with 50 ohms, the impedance WILL VARY along the line!  So it is not matched at the antenna, but may appear matched at the transceiver! Also, there is loss along the line.  I would not worry about the loss at 160 meters.
You need a matching device... either at the rig, or at the antenna connection... something to convert the unknown impedance at the antenna to 50 ohms.  That is what an "antenna tuner" does, using variable capacitors and/or coils.  There are manual and auto-tuners, or you can build one.
Good Luck!


Posts: 599

« Reply #10 on: August 06, 2008, 01:55:37 PM »

You have made your total antenna length (150 feet) longer than a quarter wave at the desired operating frequency.  This is both good and bad.  

It's good because the radiation resistance of a bent quarter wave is considerably less than 50 ohms so it would be a fairly bad match for 50 ohm coax.

It's good because you've raised the radiation resistance of the antenna by making it longer than a quarter wave and thereby improved its efficiency.  

It's bad because you've introduced additional inductive reactance by making the antenna "electrically long."  It's NOT resonant - but that really isn't a problem.

The solution is to cancel out the inductive reactance  while maintaining the radiation resistance close to 50 ohms.  You can do this by installing a capacitor in series right at the base of the vertical portion of the antenna.   This can be a variable capacitor if you wish to range over a substantial portion of the band or a fixed capacitor if you're satisfied with less bandwidth.

I have several antennas deliberately made somewhat longer than a quarter wave length.  On each I use a good piece of RG-8 as a capacitor to tune out the excess inductive reactance.  RG-8 is a lot cheaper and more weatherproof than a decent variable capacitor.  I first simulate each antenna to get some idea of the amount of capacitance needed by using the demo version EzNec - an antenna modeling program.  This gives me a ballpark estimate of the required capacitance.  RG-8 typically has a capacitance of about 30 pf per foot.   After you've estimated the required capacitance cut your piece of coax (a bit on the long side of course.)  Fasten one side of the coax to the feed point and the other to the base of the antenna.  

My understanding of the term "inverted L" is that it specifically refers to an L shaped antenna that has deliberately been lengthened beyond a quarter wave in order to increase its efficiency and raise the point of maximum antenna current higher in the air.  I always refer to quarter wave antennas that have both vertical and horizontal wires as "bent Marconis."

It seems most hams use the term "inverted L" to refer to the shape of the antenna without regard to itslength...another common example of language usage destroying precision.  LOL.
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