November 7, 2016
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I dug around in a junk box and found a TV transformer and a length of 300 Ohm twin lead. Looking at the frequencies of the local TV channels, they are spread between 506 and 674 MHz UHF. Channel 5 is on 210Mhz VHF. I am thinking of cutting a 1/2 wavelength of the twin lead cut for 580MHz and making a folded dipole out of it. I’ll place the transformer right at the feedpoint. It strikes me that this would work better than the dipole I sourced from the flea market. I’ll build one of these and do an A vs. B test over the next few days.
I may also try a coaxial dipole made from RG-59 or RG-6 coax just for grins as well. That could end up being and even cheaper version that is at least tuned to the TV frequencies.
I figure the cost of making one of the dipoles is around $5 with the transformer being the bulk of the cost. The coaxial dipole would eliminate the need for a transformer. 300 Ohm twin lead for TV is getting hard to find but I am expecting decent performance.
The loop antenna makes some sense as being better than the dipole. It has some directivity, gain and it is better matched to the TV transformer.The crossed loops that are at the flea market would make sense in that they provide a more omnidirectional pattern (assuming that they are properly constructed).
November 6, 2016
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Our local flea market. 77 flea market, has several vendors selling homemade HDTV antennas that are constructed from PVC pipe, and electrical flex conduit.The sell anywhere from $8-12 and of course the final price is negotiable. There are three versions being sold, a dipole, a loop and a “double loop”. I bought a dipole and the loop just to play around with thinking I could debunk them as any viable antennas for this application as I had already tried some store bought amplified antennas with little success.Here is what they look like:
I bought both for $14 and both came with a 8ft length of coax with a pretty decent set of connectors done with good workmanship. I took these home and connected to my TV and started a channel search. First up was the loop up about 7 feet. To my complete surprise It captured 30 stations including the ABC and CBS stations! The dipole was a little worse but comparable to the amplified antennas I had previously tried. Given this unexpected performance, I decided to look into how these antennas go together.
Essentially the dipole is two lengths of wire cut for the 800Mhz range and connected to a 300 Ohm to 75Ohm transformer. The loop is similar with a length of wire looped and connected to a transformer.Here is what one half of the dipole looks like:
I can say the workmanship is good with a solid crimp to the wire element. Clearly the wire element configuration doesn’t make much sense. When this is jammed into the PVC pipe the ends will short together and yet, there are likely some case where the wires may not short. It seems that it would have made more sense to just use a simple length of wire.
November 5, 2016
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Ok, this is not strictly a ham radio topic but it does involve aspects of radio and antenna design so I am going to post anyway in the hopes of learning a few things. I currently use cable for TV deliver along with some streaming services like Netflix and Hulu. I am frankly not happy with paying for 200+ channels when I only really watch two or three regularly. I have looked at getting free over the air stations locally through an antenna but really have not had much luck.I have tried amplifiers and amplified antennas but always found that the local channels I wanted the most were on the fringe. Here is what I have available locally as well as out a fair distance:
Here is the station detail to my QTH:
From these charts we can see the expected signals coverage, distance and bearing to each station. All but one station are UHF stations. Typically I have had issues picking up the ABC, CBS and Fox affiliates. My next post will show some surprising results with an antenna from a local source.