80M loop antenna (question)
I'm working with a fellow who has problems loading an 80M loop antenna with
ladder line and a tuner. His antenna is about 380ft and I think that it's a bit
short. I borrowed my Nye-Viking and the best that I can achieve is 3 to 1 match
at 3800 KHz.
At that point the tuner is applying MINIMUM capacitance. That makes me think
that a longer antenna length is needed.
As an aside 40m loads no better 2.5 to 1, but 30m loads up OK.
If I take his balanced line and feed it as a long wire (one end unterminated)
the system loads up just fine on 40m and 80m, but it is obviously unbalanced.
Any quick hints appreciated -- I'm not the best with HF antennas.
PS. My 80m loop antenna is 420ft long and loads as balanced just fine.
de Phil - N8PS
Actually, that loop is quite a bit LONG! 80 meters, even excluding "end effect" is still only around 250 feet!
Originally Posted by N8PS
"The more you know, the less you don't know."
Mine is cut following "the formula" so it is about 82 meters long. A bit longer or shorter is not so critical for a loop but that 380 ft is way over!
Pay attention to the length of the window ladder line though. Avoid any multiples of 1/4 or 1/2 wave. Tru to use an odd multiple of 1/8th wave.
To establish the length of my 450 Ohm window line I made up a spreadsheet with electrical lengths expressed as multiples of 1/8th wave for all wanted bands.
I chose a convenient length, fed the ladder line through a manual tuner and a 1:4 twin toroid current balun and this tunes up very very easily on all bands 80m-10m & warc. I can even tune up quite easily on 160m.
If he can hang 380 ft, why not go for a 160m horizonatl loop at 530 ft?
380 feet, especially if it is insulated is close enough to 1.5 wavelength on 80 which would be very difficult to tune. Might also explains why it tunes well on 60/30. With a horizontal loop it is best to avoid any length that ends in .5 for the band your trying to tune. Stick with around 270 feet... OR go ahead and throw up 540 feet and you can work some local/regional 160 and the 2 wavelengths on 80 would help it for DX.
Myself, I would stick with the 270 feet because the upper bands don't have as many lobes (or nulls) and if I wanted 160, go with an inverted L. To get a loop to work any kind of distance on the lower bands you need to run 2 wavelengths to overcome how low most of these things are usually strung and on 160 that is over 1000 feet. But if you can get that much up, it will have a lower takeoff angle than a dipole at the same height.
Anyway, keep it to full wavelengths whichever way you go.
Last edited by AD5ZC; 01-27-2009 at 05:12 AM.
Originally Posted by N8PS
The Nye Viking tuner FUNCTIONS as a reversible L network. L networks are known to provide the lowest matching impedance range of any type of network for a give range of tuning components. The Nye is a functional L network because the little capacitor that makes it look like a pi has too high of a reactance to make it actually work like a pi. Even if it worked like a pi the matching range would be narrow for a given component tuning range.
This is why most manufacturers settle on a T network. You should keep that in mind.
The fact it has little C and won't match doesn't tell you anything about antenna length, because the feedline interacts with the impedances and you didn't say which way the network is flipped. All it means is the antenna and feedline combo is out of range for the tuner.
As for the antenna length, an 80 meter loop should be somewhere around 250 -280 feet total wire length. Clearly the antenna is not resonant, so you do not have the only real advantage of the loop....a controlled impedance on all harmonics.
When you combine an antenna that is not resonant with a functional L network tuner you can easily get into matching range problems. The Nye is a reasonably good tuner if you understand the limitations in matching range and keep the antenna system reasonable. I would shorten the loop to 250-280 feet perimeter length and try it. The Nye will probably work OK if you do that, but keep in mind you will never come close to the impedance matching range of a T network when using a network that actually acts like an L network. It's a good tuner except for the restricted matching range.
I don't know whether it's significant in this case, but Cebik did a lot of work on "cutting formulas" and concluded that the 1005/f formula is badly out - certainly on Delta shapes. He came up with values from 1032/f to 1054/f. I've noticed exactly the same thing with my own EZNEC work.
I think ARRL have taken this on board, because the latest edition of the Antenna Book quotes 1032/f for a full-wave loop.
"In physical science the first essential step in the direction of learning any subject is to find principles of numerical reckoning and practicable methods for measuring some quality connected with it. I often say that when you can measure what you are speaking about, and express it in numbers, you know something about it; but when you cannot measure it, when you cannot express it in numbers, your knowledge is of a meagre and unsatisfactory kind; it may be the beginning of knowledge, but you have scarcely in your thoughts advanced to the state of Science, whatever the matter may be." (Lord Kelvin 1824-1907)
Look into changing the length of the feedline. I shot myself in the foot this past weekend when getting the 447' non-resonant loop up higher. Without changing the length of the loop I added 30' of ladderline to reach the new height. The result - difficult tuning on 40m.