The loop antenna

Discussion in 'QRP Corner' started by KA9UCN, Dec 20, 2015.

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  1. KA9UCN

    KA9UCN Ham Member QRZ Page

    This thread is intended to be about the horizontal loop antennas. Several hams have contacted me and or expressed interest in loop antennas.

    W7CJD suggested that a thread be started on the subject and agree this could be interesting.

    I have laid out some basics concerning this type of antenna leaving many open avenues of questioning in the hopes of building a good knowledge base around this antenna.

    With some luck this thread will spin off into other types of antenna and or QRP projects.

    With this being the first post it is a bit long winded with a lot of unanswered questions. I have put in the basics as I see them and some personal observations.

    Beginning with the basics and with interest and help from the group expanding deeper into theory and practical application of a horizontal loop.

    In its simplest form the loop antenna is 1 wavelength long on a flat circular plane. It is omnidirectional and shows no gain. It has a feed line characteristic of approximately 100 ohms in free space. It is a balanced antenna requiring no counterpoise. Polarization is determined by orientation.

    In real life conditions it can be used as a mono band antenna at 1 wavelength with no tuner given the characteristic above mentioned. It can also be used on harmonics although some retuning might be required. If the antenna is a multiple of wave length long on a given frequency it will develop lobes and nulls giving it directivity that is useful. One example would be an 80 meter loop used on 20 meters if placed in a diamond shape can produce considerable gain with the benefit of using the nulls to detune strong unwanted signals.

    This would be a subject that would be greatly enhanced by some of the antenna modeling hams on the form.

    As mentioned the loop has an impedance around 100 ohms.

    This makes a very good antenna for feeding with 75 ohm CATV type coax or 50 ohm type. The mismatch is easily tuned to well within the requirements of solid state devices.

    It can be fed with open feed line as a multi band antenna at off harmonic frequencies. This would require a tuner and again.

    This part of the topic should be discussed more in depth. The use of a tuner especially in QRP is something to be weighed. I have my thoughts on tuners but would like to hear what others think.

    In real world applications of 20 meters and down. I personally like a loop for its simplicity and performance. Notice I did not say its efficiency.

    A personal observation and another topic to be discussed is that resonant antenna are quieter with a better signal to noise ratio than at non resonant frequencies also helping to attenuate out of band noise. This property appears to be inherent in all antennas and is easily exploited.

    In the case of a loop I have noticed that often the loop is quieter. Once again I defer this to the group for further discussion.

    One of my personal likes about a low hung lower band loop is its performance as a NVIS antenna. I have never worked a 160 meter resonant loop so I have nothing to compare, apples to apples.

    On 80 meters as an NVIS antenna a full wave resonant loop shines. I suspect that the same would be true of any resonant full wave loop up to the maximum usable frequency (MUF) for NVIS work. This is particularly useful at QRP power levels were the ERP through as short a reflected path as possible can be achieved. Will greatly enhance both transmit and receive signal strength. Effectively directing a large percentage of the radiated power at a high angel to take advantage the shorter reflected path available out to a few hundred miles.

    What are the disadvantages?

    One of the disadvantages of a loop is it takes at least 3 supports to work in its simplest form.

    What are others?

    Joe KA9UCN
  2. W7CJD

    W7CJD Ham Member QRZ Page

    First off, this 1-wavelength long 80-meter coax-fed resonant loop, you like, is made of 14 ga. stranded antenna wire, at 20' height, roughly in a hexagonal shape, is omnidirectional and has no gain?

    It's qualities include NVIS and ERP?

    Fig. 2 NVIS

    (Please accept I am a beginner and experienced, both. I have read for many years, wanting to have a radio amateur license. I also have no one, locally, to answer any questions.)
    Last edited: Dec 20, 2015
  3. W7CJD

    W7CJD Ham Member QRZ Page

  4. W7CJD

    W7CJD Ham Member QRZ Page

  5. W7CJD

    W7CJD Ham Member QRZ Page

    I found this comparison of horizontal and vertical loop antennas, at the Yahoo Group Skywire Loop Antennas files section.

    It is about 20-meter loop antennas.

    Notice there are side lobes, at 10 meters height.

    It is a comparison of 20-meter Vertical vs. Horizontal Loop antennas, I have included because it has a graphic comparison to a dipole, as well.

    I would like to see this kind of thing, for an 80-meter loop.
    Last edited: Dec 22, 2015
  6. KA9UCN

    KA9UCN Ham Member QRZ Page

    Sorry I have been slow to answer. I have been down with a tooth each.

    Over the years I have had several loops all horizontal and none over 40 feet. The hexagon shape I mentioned earlier was one of convenience. Both the shape and height were determined by tree placement.

    On the coax run, I used 40 to 50 foot. Length is non-critical. This type antenna is of a low impedance type and can be used on the primary band and its harmonics I chose to use the antenna under discussion only on 80 as It was the best antenna for my situation, 80 meter NVIS. I have had ladder line fed loops but in my past situations have not seen any good reason to go with ladder line in the future.

    On gain, you are correct in that the predominant radiation is at a high angle. This would not be a good DX situation but makes for a great performing NVIS antenna. Were QRP is about getting the most transmitted power to the receiving station. A loop antenna operated at a harmonic will develop lobes and nulls. If it is set up with some thought such as in the case of an 80 meter loop tuned and operating on 20 meters. A low angel of radiation at a significant gain can be achieved. This gets a bit deep for a quick answer but a diamond shape will give the best directivity.

    On the no. 14 stranded wire, again at the time I had a supply at little more than the scrap price. Any wire that can handle the mechanical strain will work fine.

    I am not a fan of tuners and although they have a place. I would not place it high on the list of QRP equipment. The nice thing is they can be built very small and light at QRP levels.

    It would seem the yoyo antenna mentioned earlier on another thread here in QRP corner would be a very attractive alternative to using a tuner.

    I know a lot of people will not agree with my thoughts on a tuner. It is my opinion that when things are resonant the receive signal strength goes up along with signal to noise ratio. In the past I have worked a bit of QRP but for the most part was 100 to 1500 watts. You can make any marginal antenna work if you throw enough power at it.

    Joe KA9UCN
  7. W7CJD

    W7CJD Ham Member QRZ Page

    Thank you.

    I found this about 20 meter Diamond Loop antennas, at Skywires Yahoo Group.

    Is the lobe pattern really different if 40 meters or 80 meters, or, perhaps resonant frequencies only?

    My interest is to entirely avoid antennas that do not have excellent ERP.

    I like QRP.

    For me, 50 watts is "high power".

    I am interested in QRPp.

    I want to see more about antennas that make the most of that.
  8. W7CJD

    W7CJD Ham Member QRZ Page

  9. KA9UCN

    KA9UCN Ham Member QRZ Page

    That is a very good article. As earlier mentioned at one wave length low to the ground it is a great Omni directional NVIS antenna. If it is a multiple of one wavelength and in a Square or diamond shape it can produce significant gain at low angles. It can be a very good antenna with the drawback of size. The good thing is wire is cheap and trees are plentiful in most locations. If you want a low angle of radiation while maintaining omni directional properties for DX work a shape approximating a circle of several wave lengths will give the best performance. In this case there will be a lot of lobes at a lower angle. There will also be nulls but with a more Omni directional pattern. For Omni directional NVIS one wave length is about as good as it gets.

    Joe KA9UCN
  10. W7CJD

    W7CJD Ham Member QRZ Page

    Loop antenna fun.
    ON7BAS likes this.

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