Dipole Antenna Length Tests

The idea for this blog is to test the best length for a dipole antenna for TV reception. When transmitting, as mentioned in the last blog, https://blog.solidsignal.com/tutorials/cleartv-mini-antenna-tests-sort-of/ it is best to keep the antenna length to what is needed to keep the transmitter safe from being over-heated and then failing. When receiving signals, any length of antenna will work, to some extent.

What I did in this experiment was to test for the best length of a dipole antenna for two different TV stations in my area here are the questions I try to answer:

1. Is there one length that works well for all stations?
2. Can we select one length that will work for all TV stations?
3. Is it easy to determine that length?

What I mean by “work”: Will the antenna be able to receive stations well enough to be watched? This generally means a minimum of 40 dBμV on my meter. Below that, the station is usually not strong enough to insure a good-enough signal. In my area, with trees, rain, snow, and even wind, sometimes a 40 dBμV signal is not enough to guarantee a good-enough signal either.

I decided to test two channels: one weak VHF and one strong UHF. The stations to be tested are:

Channel 7 and Channel 26. Channel 7 is at 35 dBμV so I’m hoping a dipole length can be found to bring it up into the 40 dBμV range. Channel 26 is already well above 40 dBμV for all lengths of dipole I’ve quickly tested so far. Channel 26 was not being used when I wrote my last blog post. I suspect that channel 46 moved to Ch 26 due to the upcoming FCC Repack of TV Channels which is to end in mid July 2020.

The lengths tested are for the lowest frequency – longest wavelength – for channels 7 and 26. The antenna lengths tested were:
1. The 1/2-λ length for both Ch 7 & 26.
2. The full wavelength length for both Ch 7 & 26.
3. The 3/2-λ length for both Ch 7 & 26.

I tested both Ch 7 and Ch 26 at each antenna length for each channel. So both CH 7 and 26 were tested at the 1/2, full, and 3/2-λ.

To find the best single length for all stations is probably impossible for me to do by testing. This would probably best be done by a program that could simulate the antenna and modify the length and to test all possible lengths to find one that will work – at least acceptably – for all stations in someone’s area.

The answer to #3 above: It is not easy to find a single length that will work for all your stations by hand. Perhaps a program would be able to do this.

To calculate the dipole lengths for channels 7 & 46 I used the Channels vs Frequency list at:
https://otadtv.com/frequency/index.html

and then the wavelength vs frequency calculator on my website: http://cs.yrex.com/ke3fl/prgms/FreqVsWaveLength.htm

Image 1: Quarter and Half Wavelengths

Now, if we want three half wavelengths, we just multiply the above lengths by 3 to get:

Image 2: Three-quarter and Three-half Wavelengths

By evaluating my ability to actually measure these lengths, I made the decision that I’ll just use the max lengths and not try to get the other two lengths, since I think I’ll be doing well to get within +/- 2 or 3 mm. This is also because the lengths, at least, for Ch 26 are all the same length, considering my ability to measure.

My short antenna elements go from 108mm/~6.5″ to 380mm/~15″, so it can be used for the 542 MHz 1/2-λ and the full wavelength; but it can’t extend up to the length needed for 542 MHz 3/2-λ length. I’ll have to use my longer extendable antennas for all the longer lengths. This includes:

Image 3: Longer Antennas

Unfortunately, even these antennas can’t extend to the 3-λ for Ch 7 – the 2586 mm, 2.586 m or ~ 101.7″ or about 8.5′.

Image 4: Shortest Antenna

Image 5: Longest Antenna

Note: The red lines at the bottom of the window are actually underneath the smallest antenna elements I had resting on the window so that their length can be compared to that of the longest antenna tested on the pole.

 

Signal Strength (dBμV) tests for Ch 7 and 26 using 1/2, full, and 3/2 wave antenna lengths:

Image 6: Signal Strength VS Antenna Lengths

Conclusions:
One conclusion is that a single dipole length can’t receive all stations as well as the correctly-made dipole antennas for the particular station.

Another conclusion is that the 3/2-λ dipole works better for the station it is designed for, than the 1/2-λ dipole does.

Note 1: The 3/2-λ for Ch 7 works about as well for Ch 26 as the 1/2-λ dipole for Ch 26. It is, however, about 6 dBμV better than the 1/2-λ dipole for Ch 7. Also the 3/2-λ dipole for Ch 26 is also about 6 dBμV better than the 1/2-λ dipole for Ch 26.

Note 2: The full-wavelength dipole for each channel, while longer than the 1/2-λ dipole, was less effective at receiving the channel it was designed for. This also holds with the antenna theory I am familiar with. Basically, this is that the odd multiples of the 1/2-λ dipole will work better than the 1/2-λ dipole, and even multiples will be less effective than the 1/2-λ dipole.

This is seen in the results. The 3/2-λ lengths for both channel 7 and 26 were about 6 dBμV better than the half-wave dipole lengths for these stations. Also note that the 1-λ length dipoles worked but at lower signal levels than the ½-λ dipole lengths.

Earlier, I stated that for receiving signals pretty much any length of antenna will work to some extent: This statement is also born out. All lengths tested were able to receive the TV signals to some extent.

The answer the remaining questions:

1. No single length of a dipole antenna will work well for all TV stations.

2. Yes, a single length can be selected, but no, it won’t work well for all stations. However, if you are very close to all your station transmitters, then almost anything will work well enough for all stations.

I am thinking of testing different lengths for bowtie antennas to see if a longer version will work well for both the VHF and UHF stations.

Until next time, Happy OTA DTV viewing!

All my blog articles are listed at: Karras’ Corner
http://blog.solidsignal.com/author/philk/
or
Karras’ Corner Article Links
http://cs.yrex.com/ke3fl/KarrasCorner.htm
on my KE3FL web site.