Indoor VHF Delta Loop
There have been questions on one of our local repeaters about operating on 2 meters while living in an apartment. Since I am also in that position I thought I would share what I do, perhaps it will help others with a simple solution to the problem.
This is my first home brew “DIY” antenna for use on local 2 meter repeaters and simplex frequencies. The photo to the right is an example of how the antenna may be supported with minimal holes or damage to the wall. It’s modeled after the basic delta loop design and is fed with a 1/4 wavelength of 75 ohm coax (RG-59U).
This design will also double as a convenient travel antenna as it could easily be supported by a single support centered above the window. If you have a curtain rod in place that uses a mounting bracket in the center no holes are required at all. The length of wire used in a delta loop antenna is determined using 1005/[frequency in Mhz.] so a loop for use on 146.55 simplex would be:
1005/146.55 = 6.859 feet or approximately 6’ 10 3/8”.
That length can be divided by 3 for a delta loop or 4 for a quad style loop. For my delta loop that meant the wire would have to be about 27 1/2” per side.
The quarter wavelength matching transformer would be 1/4 of that length multiplied by .67 to compensate for the velocity factor of the 75 ohm cable. In this case the stub should be around 13 1/2” from the connector to the attachment point on the loop. Keep in mind that those formulas are for ideal conditions like the antenna being mounted outdoors and in the clear so plan on starting long and trimming for resonance. In my case the overall length of the loop itself was about 2” shorter per side which would probably be attributed to the close proximity of near field objects.
I don’t own an SWR Analyzer so testing was performed using an old Citizens Band SWR meter. While accuracy was questionable it did provide a reference point so the length of the loop was adjusted until I got the lowest reflected power reading
Loop antennas can be either 3 or 4 sided. If you decide to make a 4 sided loop feeding it in the center of a side element would provide vertical polarization. For horizontal polarization it would be centered in the bottom element. Attachment points are nothing more than #6 x 1” wood screws. I leave about 1/4” of the screw exposed and end up with over an inch of the screw solidly anchored into the wood inside the window opening. The hooks from the bungee cords rest behind the screw head and won’t slip off. If your in a hotel room you can fill the holes with tooth paste when you leave and they will never know…. (shhhhhh, that’s a secret). In many cases there may be a center support for a curtain rod at your window. If that is your case and there is a window sill the 3 sided delta loop design can be supported without the need for holes in the wall.
Loop antennas are usually directive off the broad side of the antenna. I was able to see a marked increase in usability over a commercially made J-Pole antenna. They can be 3 or 4 sided and polarization can be changed depending on how they are fed. I am no expert in antenna design but enjoy playing with different ideas. I think my next experiment will be to try supporting a UHF loop inside the VHF loop and putting a duplex’er inline for dual band use.
» Construction of the corners
As seen in the image to the left I used center insulators for my corners. The main reason was the fact that the groves in the insulators perfectly matched the radius in the hooks of the bungee cords that I had on hand. It also seemed to make tuning the loop much easier. By adding or removing wire from the shorting section seen you could evenly increase the length of the loop on each side maintaining form. I hope to borrow an SWR Analyzer at some time to check this method, I’m not sure what effect the metal hooks have on the antenna.
Once I determined the best length I lengthened the individual sides and soldered the shorting section in place. I used standard bungee cords to support my loop because they can be hooked over a single support centered in the window opening and the bottom two cords are hooked over the edge of the sill. I adjusted the tension on the bungee cords and the antenna sits perfectly centered on the window opening for maximum exposure. I live in a corner apartment on the top floor so I have one in a window that looks at 120 degrees and one in a window that looks at 210 degrees.
» Feed Point Connection
When delta loops are fed in the bottom corner they are considered to be “Vertically Polarized” making this the ideal method for FM simplex and repeater use. You will see in the picture to the right that I have soldered the 1/4 wavelength of 75 ohm RG-59 coax directly to the ends of the wire at the feed point. I used standard crimp on BNC connectors because I work with video surveillance cameras and had them on hand. You could substitute these for your connectors of choice. I join my matching transformer section to my feed line using a standard BNC barrel connector. If you plan on using this as a portable travel antenna make sure your feed point connection is robust enough to take a lot of flexing so you won’t be disappointed upon arrival.
Weak Signal Use
Basic characteristics of a loop antenna state that when fed at the bottom it would be “Horizontally Polarized”. With the addition of 3 more attachment points this design can be rotated so that the feed point is down and theoretically it could work for weak signal CW and SSB.
I no longer have weak signal in my “shack” since I moved my FT-100 out to my vehicle for mobile HF use. I hope to find a cheap transverter to play with and see if it works, I’ll let you know. Unfortunately in my situation my exposures are 120 degrees and 210 degrees so I hold no hope of working any aurora but those directions offer better options for collecting grid squares in the VHF contests.
There is no need to spend money for some special antenna that claims to be the best in compromised conditions. The entire cost of this experiment was $ 0.00 because everything came from my junk box. If you don’t have the parts on hand but have a hamfest coming in the near future you might plan on around $ 3.00 for connectors and insulators.