The AM Forum

Hi All,

Is the inverted L with a trap a reasonable design for limited space?

See the illustration down the page a bit:

http://www.mds975.co.uk/Content/amateur_radio_antennas.html

Thanks, Roland..........................................

Well, it appears the poor chap is terribly confused about what constitutes a good ground, good radial system, what a sloper antennas is, etc....

Unclear, but he may be loading his radial system as well.  Odd, and not necessary for ground radials...

Nonetheless, it is not unusual to load an inverted L in order to shorten the wire.  The tuning will probably be narrower.

Roland,
Perhaps someone can help with some modeling. What are the dimensions you want to fit?  

e.g.  Vertical height, horizontal run, and ending height above ground, and which part of the band you want resonant?

If nothing else, the article does give a starting point for winding a loading coil.  To avoid calculations, you could make one longer than the given design, add taps and an alligator clip and find the approximate inductance empirically.  Then use as is, or wind a more finished coil.  bill

p.s.  Note the resonant frequency can also be adjusted somewhat by changing the end height above ground.  Lower adds capacitance, higher decreases capacitance (increases inductance).

Thanks for the reply kb4qaa,

 I'm never going to have the space for a 160/80 meter dipole. Let's say the area I have available for an antenna of any sort is 40'X60'. My chimney would be the highest point at around 25' on one corner of the available space.
 I am trying to find one of those stamped steel towers that were common a few decades ago, but, most have returned to the earth by now. I don't want to use guy wires because of the space limitation. For my situation a self supporting structure would be the way to go. I could lash a supporting structure to the chimney as long as my wife gives it the O.K., seems she is deathly afraid of attracting the gods of thunder.
 Just how would I properly ground the trapped inverted L pictured?

Thanks, Roland.........................

A fiberglass mast on the chimney will help with the height.

My 80 meter dipole is short, I added loading coils to make up for about 15 feet, and that amount of loading does not cause much reduction in performance.

160 meters is out for me, but I once had an alpha delta 160-10 dipole that used loading coils, dx lb plus, and it wworked well on RX, and melted with any power...

Butternut makes a verticle for 160-40 that is self standing (and tough) that is 40 feet tall I think.
No power limits, but a good radial system is needed.

The "RCA all-band" antenna described here http://amfone.net/Amforum/index.php?topic=32897.0 is about 52 feet long. If you can come up with a modest support in addition to the chimney, you might consider it.

Thanks for all the good info guys.

The RCA antenna would certainly fit my limited space.

Where can I get additional information concerning antennas using loading coils?

Anybody know of a source for inexpensive ,self supporting towers? I don't have any tall (or otherwise) trees in my yard.

All ideas are welcome, no matter how off beat.

Thanks,Roland…………………...

Just found this:

http://n4trb.com/AmateurRadio/RCA_Ham_Tips/issues/rcahamtips1203.pdf

Roland…………………………

Now where do I find a coupler like that?

Oouupps, never mind.

It's been a loooooong time since I was forced kicking and screaming calculating complex impedance.

Can someone explain the numbers?

Thanks, Roland..............

I assume you are asking about impedances in the form  a + jx  or a - jx.

In short, a is the resistive component and jx is the reactive component. If the jx portion is positive, it represents an inductive impedance and if it is negative, is represents a capacitive impedance.

A longer explanation is available at the link below.

http://www.youtube.com/watch?v=AXl32k7AuXA


A somewhat brain dead example is if you had an antenna with an impedance of 50 + j100. Say you wanted to match it to your 50 Ohm coax and/or transmitter you'd need to get to 50 +j0. So you would need to add -j100 to the system/antenna. In the real world, you would likely add a capacitor. How much capacitance? Depends on the frequency. For 3.8 MHz, it would be 1/(2*pi*3.8e6*100) or 0.41883 pF.

You aren't going to find a coupler exactly like the one shown in the article, unless you find a homebrew one at a fest or build it yourself. But most any link or balanced antenna tuner design will work.

What's wrong with the above calculation??  Something is wrong.

Fred

Hah. Good catch. Should be 418.83 pF!

(I did say brain dead.)

I believe an inverted-L can be a reasonable antenna for limited space. The M0MTJ inverted-L installation is similar to how I installed my inverted-L at our old house, including using a chimney for support and an RF choke at the feedpoint. I used it with 16 short radials - all I had room for (I saw no mention of radials by M0MTJ). My inv-L was built by my friend K7MPP who was convinced I could get on the low bands with only a tiny yard when I had serious doubts.  The antenna loads on 40, 80 and 160 meters and includes a loading coil. Using an amp I regularly worked DX on CW, including across the pond.  I worked about 45 states but seemed to hit a wall to the Northwest.  :P

It's not at my fingertips at the moment, but if you are interested I think I can find a diagram of the antenna, with dimensions noted, and send it to you or post it here.

Whatever you decide to put up, good luck and have fun!

73,
Brad

Thanks so much for all your replies. The j operator refresher helped clear things up concerning complex impedance.

 How deep do radials have to be in sandy soil to be effective?

Thanks, Roland...........................

The less deep the better. The idea is for the RF return/displacement currents to travel through the radials instead of the lossy ground. I wouldn't bury them unless you must to avoid tangling, tripping, lawn mowers, etc. If you have reasonably thick grass, just lay out the radials, walk over them for a while to push them into the grass, pull them taught and hold them in place with some turf staples. In a month or two, the radials will be completely engulfed by the grass.

http://www.homedepot.com/p/Easy-Gardener-Steel-Fabric-and-Sod-Staples-75-Pack-815/100328706

Thanks Steve.

That RCA folded dipole reminds me of the upside down fan dipole I built in the attic some years ago. The bottom 80/75M elements were folded back just above the floor and it worked nicely. The difference was I used a current balun and coax because running the feedline through a hole in the closet ceiling and to the radio wouldn't be practical with ladder line.

I located the diagram of the inverted-L and post it here in case there's any interest. Put down as many radials as you can. It doesn't require much space and has been a good multi-band antenna for the low bands. I'll be using the inverted-L again soon now that my Apache is up and running.

73,
Brad

Just a comment here on towers. Around here at least, a ride in the country can often get some results. There are many old TV towers around that are no longer used because of satellite TV. I stop at a place that has an old rohn 20 or whatever that appears to be no longer used. If I can see a safe place to drop it, I offer to take it down for free, and will cut the legs down to the concrete. So far I have attained about 300 feet of tower for myself and others. Jut make sure you have a couple of fellers with ropes to make sure it falls in the right direction when it comes down. Saw off the legs close down, and then finish with a saws all and/or grinder after the tower is removed. Sometimes you can round one up at an old abandoned commercial building too. These towers are not suited for beams and rotors, but work great for dipoles and vee's.....

I'm curious... how do you keep a thin tower from getting bent when you "drop it?" (Rohn 20)

Sounds like you are using several gin poles with ropes to support it and let it lean over as carefully as possible?    Please describe how you do this.

T

Most of the tv towers are only two to three sections. If you cant handle a light load like that, dont do it. I have taken down plenty of these and have not used a gin pole or any of that. Nobody in their right mind is gonna climb an old TV tower anyway. My intent was to offer a supply of towers that are free, not a step by step instruction manual on how to take one down. If you are not comfortable getting one on the ground, buy one.

You sound rather defensive.    It was not my intention to infer it can't or shouldn't be done, rather, as a tower guy I was looking for more hints and kinks if and when I want to lower a small tower in a similar manner.  Sounds like you have a good method of doing it.

I have five towers here, two at 190' and one at 150'... put most of them up myself, so pretty comfortable working on them... ;)

T

Some folks fail to realize who they're talking to.

Fred

Now, wait a minute! Is this New England math :D :D
You should have never taken down all those 6 meter beams. I was hoping you would set some new DX world records.

Then apparently no "explanation" is required...

Pete said:
"Now, wait a minute! Is this New England math :D :D "



There was a post recently with a link to a ham from Arizona? who put up a series of ~190' homebrew towers. All smaller sections were welded together- making one big 190' span!  To stand them up from the ground he first had to stand up a 50' gin pole. Then this 50' gin pole raised a second set of taller gin poles. He then raised the 190' towers with these 4-5 gin poles supporting the various levels.  Incredible home-spun operation.

But the end result is he had several very tall one-piece guyed towers standing.   This guy really knew the physics of towers.  He went on to hang a number of large Sterba curtain arrays.  I'm always interested in how people get these things accomplished.


Pete - My other two towers are 110' and 60' - not worth mentioning... ;)

Is the 6M cycle still a disappointment or has it come back to life?


T

This is good. Now readers can compute correct addition  ;D

This current cycle has been a big disappointment for 6M at least in the U. S. although we did have an Sporadic E opening to 4 and 5 land several days ago. Trans-equatorial openings have popped up between Europe and Africa and Southern U.S. and South America but, given the decent sunspot numbers over the last several months and MUF predictions rolling upwards to 70 and 80 MHz at times, it's been head scratching why 6 has not come to life. It's like we're all missing something that no one can put their finger on. On the lighter side, I still think your super/super 6 meter array could have set some new 6 meter records. Did you ever calculate the forward gain of that array?

Strange that 6M defies the usual sunspots. Well, maybe there is something new to be discovered.

You must mean the homebrew sixteen 6M 3el Yagi stack?     It was eight 3el to the SW USA and eight 3el to the NE for Eu.   It started at 190' high and worked its way down the tower, fed in phase.  It was a lot of work, lot of climbing. Built and put them up by myself.

I figure the gain is what eight Yagis would make over a single one, so maybe  9dB over a single Yagi.  There are H frames out there that have more gain, esp for moonbounce.

T