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Author Topic: INV - L specs and help  (Read 24003 times)
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K4BOF
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« on: November 28, 2015, 11:11:38 AM »

Trying to mock up a very simple Marconi INV-L that I saw in an Orr book I have. 

I have a 135' wire going vertically about 4' away from my tower for 40-45' then bends perfectly horizontal for the rest of the wire length.  I have it tied off to a far tree.  The wire is 13g. 

At the base of the tower I have a tuning network that is enclosed in a box.  From the feed point into the box, the center conductor goes to one side of a large multi looped coil that I can adjust the tap to, the other side of the coil has the wire that goes vertically up the tower.  Between the feed point and coil I have a Jennings Cap that is variable.  The other side of the Cap is attached to a ground rod and several radials that vary in length.  I was going to attach a chain link fence and a hose bib to the ground side of C too. 

I am using an mfj-259b analyzer directly connected to the feed point with a barrel connector.  I have a strong AM Broadcast station close by and it is difficult to take a reading on the mfj box so I put in line an ICE BCB filter that is very effective and gets the long wire quiet to take a measurement.

When I tune in at 1.885mhz the SWR was pretty wacky on the mfj box until I attached a small jumper with an alligator clip to the ground lug on top of the mfj box to my station ground, not the antenna ground.  The reading on the mfj box with the jumper attached to the antenna ground was like 9:1 to 15.4:1 SWR.  When I attached a jumper from the ground lug of the mfj box to the station ground, the SWR was pretty awesome at 1.2:1 at 1.885mhz and steadily climbed to 2.0:1 SWR at the top and 2.0:1 SWR at the bottom of the band. 

I'm not sure if I am tuning this wire up correctly or if I should attach a short jumper from the mfj ground lug to the station ground.  I don't want to prune too much more wire either as I am thinking I am close to resonance.  The run to my shack is about 15 feet and I was thinking about placing a 5-6 turned rf choke at the feed point of the tuning network wrapped with a few ferrite cores.  I will be using LMR-240 coax for the coax run between the shack and feed point.  Am I setting this wire up correctly?  Thank you for the help.
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W1ITT
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« Reply #1 on: November 28, 2015, 12:49:03 PM »

You're on the right track.  All grounds should be connected together, station ground, the ground rod and radials, matching network box, and anything else in the vicinity that looks metallic.  And the MFJ bridge ground lug goes to that as well.  Don't let anything "float".  In short wave and medium wave high power stations, everything gets bonded out in the RF yard.  If that AM broadcaster goes to low power at sunset, you might try measuring then to prevent the MFJ from getting overloaded.
You mentioned 135 feet total wire.  An old trick with inverted L antennas, if you have the room, is to extend the horizontal section.  If you do this, you will end up using  a series capacitance to cancel out the inductive reactance, then the L-Network takes care of the impedance transformation.  But, extending the horizontal part out another 30 or 40 feet more will have the effect of raising the base impedance that your matching network sees, and this will lower your total system loss, assuming that your radial field is not perfect.
There's nothing magic about a quarter wave.  W1BB, regarded as the grand old man of 160m DXing, popularized this trick during the 60s.  First, get your antenna working as you have it, then try the extension.  If you have the MFJ telling you good things, I'd say it's time to go back to the shack and send some RF down the coax and see what comes back.
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W1ITT
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« Reply #2 on: November 28, 2015, 01:15:38 PM »

I couldn't find the copy in an English article, but here's the W1BB extended inverted L in an article in Italian.

http://cqcqdeiq2gm.blogspot.com/2012/08/160-metri-inverted-l-antenna-di-w1bb.html

Open up the second picture down and you'll get W1BB's sheet.  You can stumble through the Italian lingo and get the drift of it, but he;s doing the same thing.
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K4BOF
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« Reply #3 on: November 28, 2015, 02:15:56 PM »

Thank you for the quick response. 

I have a roll of 14g that I can use to stretch out to 144-155' and can try your recommendation of going longer horizontally.  I want to give the 1/4 wave a try first then go out to 3/8 if it doesn't tune.

Could I be having some de-tuning from my tower?  Also, I had another ham recommend not grounding anything in this circuit or antenna,...just connect to feed point and radials, then let it rip!  Not sure about this.

If I go to 145-155', how much additional wire should I add for soldering connections and insulators for tie down?  Thanks for your input!
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Jim, W5JO
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« Reply #4 on: November 28, 2015, 02:59:38 PM »

Trying to mock up a very simple Marconi INV-L that I saw in an Orr book I have.  

 The other side of the Cap is attached to a ground rod and several radials that vary in length.  I was going to attach a chain link fence and a hose bib to the ground side of C too.  


Take a look at your chain link fence.  The bottom should have a wire used to stiffen the bottom of the fence.  That wire will be tied to the end posts where another wire will be added for the next leg of the fence  At gates the wire will terminate and start on the other side of the gate.  

Make up some jumpers and get some mechanical connectors and some Noalox.  Clean the wire and put a good coating of Noalox on it then attach the jumper between the posts making an electrical connection along all pieces of that wire.  Once they are all connected you will have one radial the length of that wire.
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K4BOF
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« Reply #5 on: November 28, 2015, 03:49:24 PM »

Thank you Jim. 

I have a standard height chain link fence.  I am wondering if laying and zip tying a radial wire directly on top of the fence, a length of 132',  would that help as a counterpoise and be high enough for effectiveness?   Thank you.
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DMOD
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« Reply #6 on: November 28, 2015, 04:47:29 PM »

Here is a tuneable ATU for use on the high band that I used when I had an inverted L:


Phil - AC0OB
 

* T Tuner for Inv. L for AMFONE.pdf (22.66 KB - downloaded 184 times.)
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Jim, W5JO
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« Reply #7 on: November 28, 2015, 04:48:14 PM »

Thank you Jim. 

I have a standard height chain link fence.  I am wondering if laying and zip tying a radial wire directly on top of the fence, a length of 132',  would that help as a counterpoise and be high enough for effectiveness?   Thank you.

I did that at another location and had to keep a watch on it.  Deer and other things happened to it so it needed  repairs from time to time.  Granted the wire on the bottom is galvanized but it is sturdy and has not caused any problems.  I used crocus cloth to clean the wire first then put a generous helping of noalox on the connection.  

I am lucky in that there is no interference from BC stations here so I monitor the antenna occasionally with my 259B.  I have not seen the Z figures change much in 4 years.  The length of my wire is about the same as yours but I would like to have additional horizontal length.  About 165 ft. fed with a series cap of about 150 to 300 pf makes a good match with a good ground system.

Have you changed the 259B to the frequency counter with all grounds connected to see if it is picking up a frequency?  
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Steve - K4HX
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« Reply #8 on: November 28, 2015, 07:22:03 PM »

There's nothing original or magic in the W1BB design. Extend a 1/4 wavelength (WL) wire to 3/8 WL and the resistive component of the impedance will increase and the reactive component will go inductive. The exact magnitude of the resistive component will depend on the quality of the ground/ground system. You can adjust the length of the wire to make the resistive component about 50 Ohms. The inductive reactance will generally be around 400-600 Ohms. This would require a series capacitance of about 200-300 pF for 160 meters.

A 1/4 WL inverted-L will generally be better for DX than a 3/8 WL since it has more low angle radiation. For state-side AM operation, a 3/8 WL version would likely be better. No matter what you go with, optimize your ground system - install more radials.
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K4RT
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« Reply #9 on: November 28, 2015, 08:48:38 PM »

Hello Bruce,

With the antenna 15 feet from your station, I would not connect station ground to antenna ground.  Doing so clearly resulted in erratic operation of the antenna and/or the tuner. Your antenna or tuner seems to work properly with separate grounds.

I would never connect station ground to RF ground.  The station's grounding is for its own sake.  Generally, grounding at the antenna is for RF and in some instances for providing a path to ground at the antenna for static drain & lightning protection.  If you have a ground rod in place at the feed-point, you would want to connect the common radial lead to the rod.

In regard to building an RF choke for the feed-line, you may find that the choke is not needed with the antenna and your shack on separate grounds.  However, with the antenna only a short distance from your shack, you may be getting sufficient RF in there to cause problems, and will need a choke even with separate grounds.  Time will tell.  I seem to recall that my handbook has a chart showing the number of turns needed for a particular band.

73,
Brad
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Steve - K4HX
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« Reply #10 on: November 28, 2015, 08:57:30 PM »

A station should never have separate grounds.
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K4BOF
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« Reply #11 on: November 28, 2015, 09:36:03 PM »

There's nothing original or magic in the W1BB design. Extend a 1/4 wavelength (WL) wire to 3/8 WL and the resistive component of the impedance will increase and the reactive component will go inductive. The exact magnitude of the resistive component will depend on the quality of the ground/ground system. You can adjust the length of the wire to make the resistive component about 50 Ohms. The inductive reactance will generally be around 400-600 Ohms. This would require a series capacitance of about 200-300 pF for 160 meters.

A 1/4 WL inverted-L will generally be better for DX than a 3/8 WL since it has more low angle radiation. For state-side AM operation, a 3/8 WL version would likely be better. No matter what you go with, optimize your ground system - install more radials.

Steve-
Where would you suggest a good jumping off point in 'feet' for the wire would be?  3/8ths WL is 186ft at 1.88mHz.   Also, keep in mind tie off points, as I like to solder insulators at the very ends for rope.
Thank you.
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K4BOF
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« Reply #12 on: November 28, 2015, 10:32:12 PM »

OK, so I looked back through my Orr book. 

I looked at the W1BB antenna.
 
Total length of wire is 165-175' with 500-800pF in series. 
Yet on the next page there is a reference to the same antenna illustration that it is "bascially a 5/8 WL" which would put the wire at 316' long at 1.85mHz.

The W1BB goes vertical for 40-50' then bends over for the other 120' horizontally.  Total length = approx. 175'. 

Steve, you mentioned 3/8 WL long, which is 190' at 1.85mHz,... so which is it?

There is another 160m antenna in the same Orr book that W0BTU uses.  It has a coil and cap that works with a ground connection but only goes out to 145-155'.  I believe that W0BTU uses two caps only and not a coil.  http://www.w0btu.com/160_meters.html

I'm still kinda confused on these things. Huh  3/8 WL at 1.85mHz seems longer than the 165-175' as suggested in the Orr book?   
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Steve - K4HX
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« Reply #13 on: November 28, 2015, 11:10:15 PM »

IIRC, Orr's was 5/16 WL.

Length doesn't matter that much. Get as much vertical a you can and optimize the ground system, no matter what total length you use. That said, a total length of 160-180 feet should work FB

One other thing, make your measurements right at the base of the L. This will eliminate any ground loop induced errors. Make the wire longer than 3/8 WL and trim until you get about 50 Ohms. Then tune your cap until the reactance goes to zero. If you see much difference when making the measurements back in the shack, you either have lossy coax or an RF/ground problem. A better ground system and/or choke at the feed point should eliminate this problem.

If you go with 1/4 WL total length, you can match to 50 Ohms with a tapped coil/capacitor combination or several options of coax transformers. Or, unless your have a VERY long run of coax, just forget about the small amount of SWR and skip the matching.

Finally, don't bother soldering anything, until all trimming is done. Frankly, I would skip the soldering all together, unless you are using stranded wire. It just causes solid wire to break. Make a good mechanical joint. I do this with insulated wire and sometimes put a tie wrap and/or shrink wrap around the wire. I've never had this arrangement break lose. My dipole has been up since 2005.
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KA2DZT
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« Reply #14 on: November 29, 2015, 12:52:30 PM »

Hello Bruce,

With the antenna 15 feet from your station, I would not connect station ground to antenna ground.  Doing so clearly resulted in erratic operation of the antenna and/or the tuner. Your antenna or tuner seems to work properly with separate grounds.

I would never connect station ground to RF ground.  The station's grounding is for its own sake.  Generally, grounding at the antenna is for RF and in some instances for providing a path to ground at the antenna for static drain & lightning protection.  If you have a ground rod in place at the feed-point, you would want to connect the common radial lead to the rod.

In regard to building an RF choke for the feed-line, you may find that the choke is not needed with the antenna and your shack on separate grounds.  However, with the antenna only a short distance from your shack, you may be getting sufficient RF in there to cause problems, and will need a choke even with separate grounds.  Time will tell.  I seem to recall that my handbook has a chart showing the number of turns needed for a particular band.

73,
Brad


BAD IDEA,  one ground,  antenna ground should be connected to the main station ground point and all of it should be bonded to the AC service ground.

An antenna gets hit by lightning and bad things happen when there is more than one ground in the earth unless everything is bonded together.

Fred
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K4BOF
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« Reply #15 on: November 30, 2015, 09:35:26 AM »

The journey continues,... Yesterday, I went out and started pruning the long wire.  It was cut for 182'.  I measured twice, cut once.  The Cap is a rated at 5kV (W1BB suggests a low voltage Cap, does it matter?), it is a Jennings Vacuum Cap with an adjustable black knob at the top.  It works and I tested it with a capacitance meter.  It measured 120pF - 1000pF.

The Cap was is line when I did the pruning and I made sure it was between 500-800pF.  So I commenced to pruning in one foot sections at a time!
 
I am not certain if I should have left the Cap out to begin with?  Steve K4HX had mentioned getting the wire to 50 Ohms then inserting and adjusting the Cap (?).  How is this done?  I left the Cap in line and fiddled with the Cap until I could see the lowest SWR on the MFJ box.  Stupid MFJ box goes into the sleep (SLP) mode WAAaay to often, hate it!  I was running it on fresh AA batteries to keep the AC wall wart out of the configuration. 

Anyway, I pruned and kept adjusting for lowest SWR at 1.800mHz on the MFJ box.  I left the MFJ box set at 1.800mHz.  I checked all of my connections with a Ohm meter to be certain.  I had several multi length radials and six grounding rods connected, including my tower and station grounding rods.       

Around 163ft-165ft I started to see some readings on the MFJ.  At 1.766mHz (the lowest the MFJ box will go), I was seeing 1.7:1 SWR.  With any Cap adjustment, I could not get a 1:1 match.  From 1.76mHz and up, the SWR rose steadily.  I fiddled and played with the Cap throughout the 160m band and really could not get the SWR lower than approx 2:1 SWR,...it was never flat at 1:1 anywhere on the band.  Maybe, I am not performing the procedure properly?  Call me a dunce, but I kept on cutting the wire (too long), thinking that I was out of resonance as the SWR was lower when I went lower in frequency on the MFJ box (again, wire too long?).  I finally got down to approx. 155ft and saw a decrease in SWR again at 1.766mHz (1.5-1.6:1).  Again, thinking that if I go shorter in wire length it would bring the SWR to a comfortable level in the middle of the band,...well it didn't.  I got a little frustrated at 145ft, and stopped pruning there. 

I adjusted the Cap to 50 Ohms at 1.900mHz, didn't catch the X number.  The SWR at 1.900mHz was 2.0:1.  At 1.800mHz it was 3.2:1 SWR and at 1.999mHz it was 2.8:1 SWR.  I could never get it flat. 

When I reconnected my 15ft run of LMR-240 into the shack, there was a shift in SWR.  The wire ended up being resonant at 2.15mHz with a 2.0:1 SWR.  Going above or below this frequency showed an increase in SWR.  I am thinking that the wire is now way too short and out of resonance at 142ft now.

Am I to believe that maybe I should have left the MFJ at 1.900mHz and tuned the cap for best SWR there?  Any continued assistance is appreciated.  Maybe I should re-connect a long piece of wire back on to what I have to make it 165-175' again and then re-tune the MFJ at 1.900mHz and adjust Cap for lowest SWR?  Not sure.  I guess I could adjust the Cap for lowest SWR in the widest part of the band and let it creep up steadily on either side of the frequency? Huh     

   
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« Reply #16 on: November 30, 2015, 01:54:49 PM »

You are to believe  cap should not be in line until your wire is 50 ohms.   Then,  put the cap in line to cancel out inductive reactance.

The way your doing it,  you very well could have brought the antenna through  50 ohm point,  and beyond.

Adding coax from the feed point to the point of measurement is going to move you around the Smith chart  ,  but you still have standing waves at  feed point.  IOW,   proper way to do it.

You said random length radials as well....   You might have too much ground loss preventing you from ever seeing 50 ohms.   IE if your antenna is 36 ohms,  and you have 36 ohms of ground loss,  you'll never be able  to get to 50 ohms.   72 will be  best you're going to see,  if I remember correctly.

--Shane
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K4BOF
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« Reply #17 on: December 01, 2015, 11:05:51 AM »

Yes, I think that I missed the 50ohm point. 

Guess, I'll try a 'KA7TRP shunt tower effect' since I cut the wire too short and add a coil.

I can re-solder wire to what Ive cut and try again, too.  Thank you. 
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K4BOF
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« Reply #18 on: December 01, 2015, 03:41:25 PM »

Procedural question:

If I go to 145-155' or even 165-175' of long wire, should I FIRST attempt to measure resistance (R as close to 50ohms as possible) with the MFJ box and only the wire (no Cap inserted) within the desired frequency I would like to work. 

Then secondly, prune the wire to get it close to or equal to 50 ohms at that frequency (only the wire). 

If and when I get it close or equal to 50 ohms, I then insert the cap and tune out X = O with only the cap.  I appreciate the responses.  Thank you.
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Steve - K4HX
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« Reply #19 on: December 01, 2015, 03:55:07 PM »

That's how I would do it.

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K4BOF
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« Reply #20 on: December 06, 2015, 01:19:55 PM »

Another update,...
Great Wx here in VA, so got back outside yesterday to re-solder some more length onto the shortened wire. 
Was previously down to 139.5' so I re-soldered on another 36' to try the process again,... this time without the C in line.  I could never get that wire to 50 Ohms!  It was either too high or too low in R floating around 36 to 40 then off to 70 with cruddy SWRs, nowhere near a 1:1 match.

I finally decided to try KA7TRP's trick and attempt to shunt feed the tower and push in an inductor / coil.  So,... I started pruning again!  Finally seeing a consistent 50 Ohms right in the middle of 160m band with X=0.  Success!

I measured at the feedpoint and decided to slip in the coax and read the SWR at the shack, too (using LMR-240 Ultraflex) with the MFJ box.

At present, I have NO Cap and NO Inductor / coil in line.  Here is my data on my INV-L inside the shack with my coax run:

1.800mHz  R-100 X-69  3.0:1 SWR
1.825mHz  R-116 X-9   2.3:1 SWR
1.830mHz  R-113 X-6   2.2:1 SWR
1.850mHz  R-84  X-24  1.8:1 SWR
1.875mHz  R-60  X-18  1.4:1 SWR
1.900mHz  R-51  X-2    1.0:1 SWR
1.925mHz  R-50  X-12  1.2:1 SWR
1.950mHz  R-55  X-23  1.5:1 SWR
1.975mHz  R-67  X-33  1.8:1 SWR
2.000mHz  R-91  X-34  2.1:1 SWR

Does this look about right?  I laid down a 132' radial that is 12'-15' off the ground running directly below the horizontal section of the INV-L. 
Thank you for the recommendations and assistance.
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KB2WIG
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« Reply #21 on: December 06, 2015, 06:59:15 PM »

B

                    "re-solder some more "

Like, get  yer mojo together and use the Split-bolt. Women will swoon, and men will talk to you with respect.

Do it today.

Do it for the chillin.

KlC

Its also a lot easier to make changes, and you lower the chance of burning yer self with your American Beauty.


* hangin with the Big Cu.png (17.24 KB, 100x100 - viewed 489 times.)
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K4BOF
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« Reply #22 on: December 06, 2015, 09:33:22 PM »

That bolt is sweet brutha.  If it'll make the ladies swoon, I'm on it.  ; )
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KB2WIG
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« Reply #23 on: December 06, 2015, 09:49:03 PM »


B,

Yess, you too can make a righteous swoon with the ladies. The Derb schooled me to the groove.


klc
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« Reply #24 on: December 07, 2015, 01:06:19 PM »

[...]

Does this look about right?  I laid down a 132' radial that is 12'-15' off the ground running directly below the horizontal section of the INV-L. 
Thank you for the recommendations and assistance.


I recommend two symmetrical elevated radials instead of just one. That way they will not radiate. A horizontal radiating element that close to the ground will contribute to inefficiency. (Having lots of ground losses makes your SWR bandwidth wider, by the way, at the cost of making your signal weaker.)

Two 100' long elevated symmetrical radials would work well. They don't need to be a full quarter wave. Even two 50' radials would be almost as good. The resonance of the antenna will change as you vary the radial configuartion, though. I use a tuner so resonance is of no consequence to me -- only pattern and efficiency are.

73,

Kevin, WB4AIO.
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