Friday, December 6, 2013

Simple Ham Radio Antennas--a multiband indoor loop antenna. Post 247

Over the past few posts I've been recounting the joys of erecting antennas with no space restrictions.  On my new property in the Puna District of Hawaii Island, I have an acre of land with few close neighbors and a comfortable distance from the Keaau to Pahoa Highway and all of the power lines following that road.

However, for most of my 36 years as an amateur radio operator, I've had to erect compromise antennas because of space limitations, proximity to high voltage power lines, and the eyes of suspicious neighbors.  Most of these antennas worked very well, considering the space restrictions of my rental housing.  One thing I did discover is just how good a basic 1/2 wave length horizontal dipole or inverted v performs when you use a moderate length mast (33 ft/10.06 meters) coupled with 450 ohm feed line, a 4:1 balun, and a decent transmatch.  This combination gives you multiband capability with the design frequency being the lowest band you wish to use.

I've also used a variety of indoor antennas, ranging from the commercially bought MFJ-1622 vertical with tapped coil and counterpoise to homebrew loops fed with 300 ohm tv twin lead or 450 ohm ladder line.  I was able to get many contacts with these antennas, despite their small size and proximity to electronic devices in my former homes.

While I'm moving to my new location (it will take a few months in between teaching assignments), most of my backyard antennas at my Laupahoehoe qth have been lowered, packed away, and taken to the new home site.  The under-the-house 40 meter loop is still available for local and statewide contacts.

Until I get all of my antennas erected at the new location, I'll have to make do with some temporary antennas.  Since I've had good results with HF loop antennas, I decided to make another HF loop to supplement the existing loop under the shack.  This time, I put the loop inside the house, tacked to the ceiling of the living room and fed with 450 ohm ladder line into a 4:1 balun connected by a short piece (3 ft/0.91meters) of RG-8X coaxial cable to my trusty Drake MN-4 antenna transmatch.  For reasons of rf exposure and safety, I decided to use my portable qrp rig (Yaestu FT-7) as the transceiver.


Using valuable information from "Yukon John" (KL7JR) and Scott (K2ZS), I decided to use an 80 ft/24.39 meters loop fed by ladder line into a 4:1 balun and a sturdy antenna transmatch (Drake MN-4).  The antenna would give me 20 through 10 meter coverage at minimal cost and reduced rf exposure.

I had some spools of #22 AWG hookup wire in the garage, a package of  push pins in the shack desk drawer, 10 ft/3.04 meters of 450 ohm ladder line in the desk drawer, a W9INN 4:1 balun on the workbench, and the Yaesu FT-7 in the van.

In his article on stealth loops, K2ZS suggested a length of 70 to 90 ft/18.29 to 27.43 meters for the indoor loop.  I chose a compromise value of 80 ft/24.39 meters because that fit the dimensions of my large living room.

Using a small ladder for support, I used push pins to attach the loop to the living room ceiling.

The 450 ohm ladder line was soldered to each end of the loop.

The ladder line was dropped straight down to the floor.  The ladder line was connected to the W9INN 4:1 balun.  Three feet/0.91 meters of RG-8X coaxial cable with UHF connectors was attached to the balun and run into the Drake MN-4 transmatch.  Short patch cores made from RG-8X coaxial cable (about 2 ft/0.06 meters long) were used to connect the dummy load, low pass filter, and transceiver to the antenna transmatch.

To minimize rf interference to our home computer and entertainment systems, I only operate during the afternoon or late evening when our use of the entertainment equipment is minimal.  I also keep the power below 10 watts and operate primarily CW.  So far, I've encountered no rf problems in the house.  I do have a good supply of torroids to use on power and computer cables should the need arise.


The indoor loop works very well on 20 through 10 meters.  My CW reports range from 549 to 579 depending on propagation.  On the occasional SSB contact, I get reports between 54 and 57.  My low power signal won't break a DX pileup, but, for casual contacts and local rag chews, the indoor loop is adequate until my final move out to the Puna District.

Best of all, I don't need a counterpoise to run along the baseboards of the floor. 

I'll try out this indoor loop during the upcoming ARRL 10 meter contest (14-15 December 2013).

This indoor antenna was fun to build and cost me practically nothing.


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Aloha de Russ (KH6JRM).

BK29jx15--along the beautiful Hamakua Coast of Hawaii.