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Author Topic: Marconi or L for low band operations  (Read 7300 times)
KT4NR
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Posts: 571




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« on: September 17, 2009, 11:59:04 AM »

I am finally into the new house. This fall/early winter I will be doing some antenna farming. The basic idea is to do something for HF that will include getting on Top Band plus 10-80.

The lot: I have a rectangular lot about 75 ft wide by 200 to 300 feet deep. It is roughly .5 acres total. All the trees are at the back property line. The house is fronted as this is the flat spot in the property. There is a steep hill going down about 25' from where the house is to where the hill flattens out. Antennas will have to run from one of the 2 chimneys to the trees at the back of the property. Since the trees are all mature they are 50+ ft tall so at least as high up as the chimney if not taller. (Remember house is at the top of the hill.)

Wire antennas would be run out from the chimneys so I can stay away from aluminum siding.

Beams are not allowed for HF nor is a Tower..... for now.... (wife not the neighbors on that one)

Current ideas:

- A 160m loop fed with 450 ohm twin lead (one antenna to do it all)

- A marconi or L antenna for 160 plus a 10-80 dipole or V

I enjoy DX'ing and also rag chewing stateside. I do realize that I will not have the feedpoint at an optimal high for much of anything except 40m and up. I also run QRP or 100w (no QRO sorry) and have been thinking of playing with more digital modes.

Based on my reading the L or Marconi will both accomplish the same thing on the low bands but there is a difference in which one needs radials and which one will do better for DX versus NVIS. So I am just seeking some input on what people have found with their Marconi or L antennas and whether they would be better than using the loop under the aforementioned circumstances.


Thanks for any input.

Dan S
KO1D
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N3OX
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« Reply #1 on: September 17, 2009, 12:50:35 PM »

"Based on my reading the L or Marconi will both accomplish the same thing on the low bands but there is a difference in which one needs radials"

I don't know how you're defining "Marconi" but my understanding is that a "Marconi" antenna is a wire fed against ground, in which case an inverted L is a special case of a Marconi antenna.

If you're talking about this twin-lead Marconi thing that seems so popular, and someone is saying that it requires fewer radials than a regular inverted L of about the same physical length, they're just wrong.  There's a common misconception that "folded" monopoles, since they have a higher feed impedance, have less loss for a given loss resistance, but that's false.  The feed impedance of a folded inverted L type antenna contains equal proportion of resistive part going to radiation and resistive part going to loss... they both get transformed up.

If you had a 1/2 wavelength wire fed against ground, that doesn't need quite as much of a radial system, but a limited radial field will stabilize the feed impedance.  But in most installations, a half wave inverted L would radiate about the same as a low dipole with no benefit for DX, because the vertical bit has very low current.

So a good end-fed wire for 160m needs some sort of radial system for decent performance.  You can't really get around that.

And an inverted L is an excellent all-around antenna for 160m.  You can get some DX and the horizontal bit gives some good high angle fill in for better local performance.  The twin lead Marconi idea does step up the sometimes low Inverted L impedance closer to 50 ohms, but there are other ways to do that easily.

Something to remember about 160m is this: if it sounds too good to be true, it probably is.  If it saves you radial-installation sweat, it's probably sacrificing performance.   There are a lot of people who try some thing or other, run 1500W to it, and are able to work lots of 160m DX so they conclude "oh, I can work DX with it so it can't be THAT bad."

But I have worked 116 countries so far from the DC suburbs using 100W to an 1/8th wave vertical over a scrawny radial field.  I estimate the efficiency vs. a full size quarter wave to be something like 30%.  So with 30W ERP with respect to a huge vertical, I've worked more than DXCC's worth of DX on 160.  That says something about the people who need 1500W to do that with their weird "special" radial-less contraptions .  :-)

An inverted L fed against good radials is an easy, proven, no nonsense Topband antenna.  If you put up a good solid inverted L with 50 or 60 feet vertical and the rest going out horizontal or at a shallow angle, and you put in a decent radial system, your 100W will get you all over the country and a good bit of DX on 160m.  

73
Dan
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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
AA5TB
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« Reply #2 on: September 28, 2009, 11:57:39 AM »

Hi Dan,

I'm in a similar situation.  I use an inverted-L that is fed at the base via a SGC SG-237 autotuner.  The antenna is a half wave length long on 40m and I only have a 8' ground rod.  I work all bands with it but it performs the best on 40 m.  I regularly work DX with 5 Watts or less and often with just 2 W.  The vertical portion is only 20 feet tall but I suspect that there is just enough current near the top of the vertical section to give me a little more low angle ERP then a low dipole.  In fact on 40 m it performs a lot better then it has a right to.

On 80 m and 160 m most of my power is used to heat up the earth worms since I have no radials.  I measured the earth resistance on 80m to be between 30 and 60 Ohms!  On 160m I consider the antenna just a leaky resistor but I have several awards on the wall for that band using QRP power so a little signal from a city lot is better then no signal.  However, my best QRP DX on 160 m is only Galapagos Island from north Texas.

On 20 m through 6 m the antenna develops more and more lobes as you go up in frequency but the performance is comparable to about any dedicated dipole that I've compared it to.  When you're stuck to having low antennas you won't notice much difference between them.

With a good radial field your results should be much better then mine, at least on 80 m and 160 m.

Inv-L Advantages:
1. Simple.
2. Inexpensive.
3. Keeps you active on all bands.
4. Works well for receiving all the down through VLF.

Inv-L Disadvantages:
1. Receives every device in your house if it is close to it.
2. Low band performance dependent on radial system.
3. No tower to show off.
4. Not compatible with AT&T Uverse. But then again, what is?

Good luck.

73,
Steve - AA5TB
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KI4VEO
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« Reply #3 on: November 24, 2009, 10:49:07 AM »

I'll be the dissenting voice, thus far, and offer my experience with the loop.  I have two - a vertical inverted delta and a horizontal.  Both are fed with homemade open wire feedline.  The vertical loop offers a better matching range for my Yaesu FC-102 tuner with 450 ohm feeder and the horizontal seems to prefer 600 ohm.  Both using remote 4:1 current baluns fed with coax about 10 feet outside the shack window.

Until earlier this year, I used no amplifier - just the 160 watt output of my FT-102 and made numerous contacts into Europe and South America.  The addition of the Heathkit SB-1000 has increased my ability to dine at the adult table.

The vertical loop favors N/S while the horizontal loop...?  Who knows.  Although the horizontal was the antenna I was using early one Sunday morning when I worked an Australian station.

For top band, a sloping vertical is not bad, if you can put down sufficient radials to limit ground losses.  I've used 160M loops with good success...albeit with a bit of difficulty with 40M because of the tuner.

You might also consider a fan dipole.

For your over-the-tree-top support lines consider nylon trimmer line, but only the round variety.  The square and Vee shapes are a bit aggressive on tree limbs when the tree sways and will damage or cut thru support branches in short order.

Pulleys and counterweights are unnecessary as the trimmer line is quiet "stretchy."
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KH6AQ
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« Reply #4 on: November 30, 2009, 05:13:57 AM »

with a height of 50' the 1160 meter inverted-L has a higher radiation resistance and will be more efficient given the usual amateur ground.

A 50' vertical has a radiation resistance of 4 ohms while a 50' x 85' inverted-L is 13 ohms.

The inverted-L has a higher radiation resistance because the current is uniform along the vertical length (the part that does most of the radiating) while the current along the vertical decreases linearly with height.

Given a ground consisting of eight 60' radials you might expect a ground resistance of 20 ohms. The vertical input R is 24 ohms and the inverted-L is 33 ohms. 33 ohms is high enough to drive directly from the radio.

The radiation efficiency of the vertical is 17% while the inv-L is 39%.
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KA3TKZ
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« Reply #5 on: December 01, 2009, 01:32:45 PM »

Put up a great big LOOP!  You have the room and a horizontal loop works well.  You couls also put up a half wave dipole 250+- feet.  Just get these antennas up as high as possible.  Get metal in the and get on the air.  I hope to work you soon.


Whitney
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N2EY
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« Reply #6 on: December 04, 2009, 07:37:23 AM »

If I understand the layout, you have a narrow, deep lot, with the house at one end and nice trees at the other. You can also attach to the house.

What I'd do in that case is to start off with an inverted L, end fed. If possible, I'd get the vertical section away from the house a couple of feet or more, and have a remote tuner and ground rod where the vertical section reaches the ground.

I'd make the horizontal section of the L as long and high as possible.

I'd also install as many radials as conditions allow. They can be whatever length is convenient; what you're really trying for is less ground loss, not resonance.

The resulting antenna will be good for 160-10 *if* the tuner can match it easily and *if* you keep the connection from the tuner case to the ground system very short in terms of a wavelength. If some bands are a bit hard to match, change the horizontal section length by a few feet and try again.

The problem with a loop on your lot is that it will be a long narrow loop, which is fine for the higher bands but not so hot for bands where the narrow side is less than a quarter-wave. It also takes twice as many high supports, which may be an appearance issue.

73 es GL de Jim, N2EY

73 de Jim, N2EY
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