Homebrewing Full Wave Loop Antenna for VHF

 I have been hearing about the low noise level for loop antennas for a long time and always wanted to try out one. Two problems faced by me were the limited space available at home and the lack of a 4:1 matching transformer needed for a delta loop antenna. After a long period of brainstorming, I decided to try out a 2m full wave loop antenna as that is the one which can easily fit in on my small first floor terrace. An online search brought me straight to the Full-Wave Loop Antenna Length Calculator. There is a lot more information on that page than just a calculator for the length of the antenna. It has been mentioned that even though a circular loop is ideal, squares, triangles and other shapes also can work well, provided you make the area enclosed by the loop as big as you can. That sounded nice and gives option to experiment with multiple shapes. A full wave loop can be used on frequencies higher than the design frequency as well. So I can use a 2m loop for 70cm when I am trying to work Amateur Radio on International Space Station, at least for the high elevation passes. Let us see!


Calculated the loop antenna length for uplink frequency of International Space Station, 145.990 MHz. It came to 2.10 m. Checked for the transmit frequency of our local Tirur VHF repeater. That came to 2.11 m. As usual, the advice is to make the antenna a little longer and then trim to get the best SWR value. The next hurdle was the balun. The calculator page had a solution for that as well. Feedpoint impedance of the design frequency is about 100 ohms. Matching can be done with a piece of 75 ohms coaxial cable one quarter wavelength long, shortened to compensate for the velocity factor of the cable. The page also gave a length of 0.34 m for a 75 ohm coaxial cable with a velocity factor of 0.66, for polyethylene dielectric. If the dielectric is foam with velocity factor 0.8, it becomes 0.41 m. Wikipedia mentions 0.75 as velocity factor for RG6/U with polyethylene foam dielectric. U in this case stands for Universal or general Utility. In that case it will be 0.39 m, for the Tirur repeater uplink frequency of 144.85 MHz. I starting with that frequency for the loop antenna as it will be longer than that needed for Space Station and I can trim it later. Moreover, testing is easier as Tirur repeater is always available while Space Station is not.

Formula for calculating full wave in meters is 306 / frequency in MHz. Quarter-wave matching section length in meters = 75 * cable's velocity factor / frequency in MHz. Feeder with parallel conductor lines will be best when using antenna on multiple bands. That is another project which I have been toying about and shelving for quite some time, mainly because I do not have an external antenna tuner which they say is a must for matching a ladder line.

Comments

Post a Comment

Popular posts from this blog

What is LMR SDR?

Image
See also (new post):   Standalone Versions of LMR SDR!  I have been hearing about LMR SDR during discussions among homebrewers for the past few days on 40m. Thought of learning a bit about it and my web searches told me that some of the persons behind the development of LMR SDR are YE3CIF, UT3MK and F5NPV. F5NPV has given detailed descriptions about homebrewing LMR SDR on his webpage, including videos. Even though I did look at the pages of YE3CIF and UT3MK, they seem to be in regional languages. I could not find the expansion of LMR on those pages, though I presume that it would stand for land mobile radio. LMR cables which many of us use were initially designed for land mobile radio. SDR stands for Software Defined Radio like my FT-710, which uses software to do many of the functions done by hardware in the conventional superheterodyne radio receiver . It is interesting to note that LMR SDR covers from 160m to 10m band, that is the whole amateur radio HF bands. There are a few frie
Read more

Building a Tape Measure Yagi for LEO Satellite Operations

Image
 The Moxon Yagi which I have been using for my LEO Satellite operations had a Moxon rectangle for VHF and five UHF Yagi elements which were passively coupled to the VHF Moxon driven element. The gain was not that high when I thought of working SO-121 QRP satellite. VU2TEK had good success with his Tape Measure Yagi which motivated me to go for one. But I don't expect the same success as he was operating outdoors, pointing the Tape Measure Yagi in the direction of the satellite pass. I am planning to use only in a mast mounted fashion which will limit its usability for satellite passes to a short period of about 4 minutes. I have chosen the DK7ZB design which I have been looking at for a quite a long time. That design has 5 elements for VHF and 8 elements for UHF with a boom length of about 1.5m. First step was to get a wooden reaper to use as the boom. Earlier it used to be available in all electrical wiring accessories shops. But now with the usage of PVC channels for wiring, I w
Read more

Six Button Wireless Mouse vs Three Button Mouse

Image
 When I started using computers long back, mouse had only two buttons, a left and a right button. Then came three button mouse with an extra scroll button. I have read that there was also a single button mouse, but I have not seen one. Now here is a six button mouse, that too a wireless mouse with a USB-A Nano Dongle and 2.4 Ghz wireless connection. In the earlier era, mouse had a rolling ball which would gather dirt and sometimes stop functioning well. Now we have optimal mouse which uses a red light source and an optical sensor instead of the rolling ball. The wireless mouse works on a single 1.5 V, AA battery with a projected battery life of one year!
Read more