Published 25 Sept 2013
AXINO TECH HOME

Re-purposing that old VHF TV antenna
Convert it to FM & DAB

Analogue TV is destined to be switched off in our area on 29 September 2013. All terrestrial digital TV broadcasts are on the UHF band in New Zealand, so the VHF aerial is now not needed for TV and could be removed. For most people, removal and scrapping their VHF aerial is likely to be the only course of action because of the weathered condition of the aerial. My aerials are in the roof space, aimed through the gable end of the house, directly at Mt Kaukau, the primary local broadcast station in Wellington. This means the aerials are in good condition and re-purposing the VHF one to be FM was feasible. Any outdoor VHF Band 1 (low band) or combo aerial, if in good condition, could also be converted to FM.

typ TV antennas pic

My VHF aerial is a 'Strongline' type which is a combo VHF comprising a 3-element Channel 1 and 5 element Band 3 with their dipoles coupled by short rods. It is fairly typical of the aerials of the time. The pictures show some common ones in the area. Some of the VHF low band elements in these examples are in poor condition and are likely best scrapped. The lower picture is a hybrid VHF/UHF and since UHF is still required, will need to be retained, although the Band 1 elements could be removed without affecting the UHF performance.

Below is a simple sketch of the Band 1 (low band) elements only of my aerial. I have not shown the shorter Band 3 elements in front since these are not changed in any way. The dimensions shown are the before (originals) and the new in parentheses after cutting them for FM. In addition to shortening the elements, the spacing between them has to be reduced also, but I only remounted the reflector element and left the director at the original spacing as the incremental improvement proved to be minor. yagi sketch

The Strongline is fairly easy to modify. Each element is in two halves, including the folded dipole. Screws pass through a clamp, through each element rod and are tightened with wingnuts. So it was just a case of removing each element half, cutting to half the new dimension shown in the sketch minus 1cm. (The 1cm allows for the diameter of the boom when reassembled) The dipole screws also pass through the connecting rods to the Band 3 dipole (in my case). I did one element at a time with the aerial still in situ because I wanted to measure the incremental changes each time, but normally, one would remove the aerial, make the changes and re-mount. The process would take less than an hour. In fact the director and reflector can be trimmed without removal. Of course, anyone working on their roof needs to take a lot of care to stay safe so might extend the time a little. Below are some pictures of the Strongline aerial clamp arrangements.

reflector clampdipole clampold and new dipoles

The last picture shows old half dipole alongside the cut down version. It's close to half the length! That makes sense because the original low band part was cut for around 50MHz operation and I made the new dimensions for 98MHz operation. Twice the frequency; half the wavelength. The dimensions are not that critical; a couple of cm either way will make no measurable difference. The only tool I needed was a drill, for new holes in the dipole ends and to drill a new hole in the boom for the reflector. When the aerial was finally remounted to vertical polarisation I also needed a screwdriver and spanner. Others might need more tools and WD-40 depending on the aerial type and condition.

Polarisation for FM

FM polarisation started out here following the polarisation used for TV in a given area, meaning it could be horizontal in some places; mostly higher density places like major cities, and vertical in others; mainly rural areas. However, broadcasters soon worked out that vertical is better for portable reception and also in vehicles, so began rethinking the plan. These days, most NZ broadcasters, except RNZ prefer vertical polarisation. For legacy reasons, most currently use mixed polarisation, meaning that it matters little which way you orientate the FM aerial. However, low power stations will almost always use vertical.

Most people don't care about antennas for FM anyway; preferring to use just a piece of wire out the back of the receiver, or at best a ribbon dipole tacked to the nearest wall. However there are some good reasons for using a proper antenna. In weak signal zones an antenna will make the difference between stereo and mono, or improve a station quality, reducing hiss due to low signals. FM reception can be compromised by reflections (ghosting in analogue TV terms). Severe multi-path (The station's signal arriving at your receiver by two or more paths, each delayed from the main path) results in audio distortion and in crosstalk. There are mathematical approximations to calculate this distortion and the general effect is for increasing distortion as the delay between primary and secondary signal increases and also for where the delayed signal(s) are relatively strong compared to the primary. The worst case of crosstalk occurs from the SCA and RDS subcarriers into the audio channel (and vice versa). Very few people really notice these impairments, especially using portable devices, and even less because of the high audio compression on commercial stations. Only people who listen to stations like Concert FM in a quiet residential environment using good receiving equipment are liable to observe audio distortion from these causes. Any given antenna has more directivity (resistance to reflections), when mounted horizontally, than it has when mounted vertically. So, for those who need to try and reduce reflections from nearby hills, horizontal polarisation is the way to go. Unfortunately this advice is at odds with the broadcasters; many of whom prefer vertical. If the local transmission of importance uses mixed polarisation, then horizontal mounting is best. However, if the transmission you want is purely vertically polarised, you are probably going to need to mount your aerial vertically; it will likely be the better compromise.

In my case, the aerial was originally horizontal. For this exercise, I made signal measurements of all 22 stations receivable around here; first with horizontal and then with vertical mounting after I had modified the aerial to be FM. The mean signal difference between horizontal and vertical was +3.5dB (decibels) in favour of vertical. Most stations improved by between 1dB and 3dB, which is hardly significant, but Coast FM went up 15dB when the aerial was mounted vertical, giving a clear indication that Coast is transmitted with only vertical polarisation.

My results

After my aerial was cut down to FM size and re-mounted vertically, the improvements were significant compared to the FM signals from the original TV horizontal aerial. Now I can connect the FM tuners to the 'TV distribution' around the house, and ditch the unsightly ribbon dipoles hanging off the walls. In situThe new vertical FM aerial in situ can be seen on the left. In fact, since I left the Band 3 VHF part alone, it is really now an FM & DAB antenna. Although DAB in New Zealand has no commercial impetus, the major transmission provider; Kordia, has had a 1 year test transmission running since 2008, trying to stimulate interest. I can just receive the test transmission from Lion House in Wellington CBD (shifted to Mt Kaukau in Nov 2014). Since making the antenna vertical, the Band 3 DAB signal increased by 8dB. The aerial you see on the right of the picture is the horizontally polarised UHF for Freeview.








Below is a table of before and after RF signal level for all 22 FM stations within range. Results are colour coded such that green means sufficient for good stereo; orange is either stereo with hiss or good in mono and red means rather too poor for entertainment value, even in mono. The units are in dBm; a unit of power. To obtain levels in dBuV, add 109, assuming 75 ohm impedance. As you can see, the improvement is significant. The mean improvement is 19dB across all stations; the least improved gained 7dB and the greatest improvement was +24dB.

RF levels table

Your results will vary from these due to your location with respect to the transmitter station, but the underlying trends will be similar.

Doing it by increments

As an exercise, I progressively converted my aerial, making measurements of all the stations with each change. Some people may find this of interest, especially if they cannot make all the changes to the aerial due to it's mechanical condition. Starting from the original state of being a horizontally polarised, 3-element, Band 1 TV antenna.

Change madeResult/comments
Step 1: Remove original directorMean +2dB increase although two stations went down. Antenna now acts as 2-element only. Remains H pol. Increase due to original director acting more like reflector at FM.
Step 2: Added cut down director in original positionMean change from step 1 is only +1dB.
Step 3: Change the dipole (driven element)The mean increase is +10dB from step 2. This step is the most significant single change.
Step 4: Replace the reflector both new length and positionThis gave a 1.5dB increase on average from step 3
Step 5: Mount the antenna verticallyThis gave on average a +3.5dB increase in signals over step 4. However improves things much more for purely vertical transmissions.

For best bang; minimal effort, I would remove the director altogether and also change the dipole (folded) element. Whether you then proceed to change a horizontal mounting to vertical mounting depends on how many of your stations are purely vertical, as opposed to mixed polarisation and whether you have the additional mounting hardware to make the change.

Notes

In many cases, the coaxial connections from the VHF antenna and the UHF antenna go into a diplexer usually a small grey plastic box on the pole. The diplexer combines the two signals into one. The common coaxial cable then heads into the house to feed the TV's. You don't need to change this diplexer when converting the VHF antenna to FM as described. That is, assuming it is still working normally and not a corroded blob of components inside. These diplexers will handle FM frequencies as well as they did TV channels. If, however, there is an antenna amplifier in line, then this may need to be checked to see if it passes FM because some of these included FM traps to try and avoid overload.

Astute readers will note I use both terms aerial and antenna within this document. They are the same thing. Most people call them an aerial but the correct technical term is antenna. The plural form is NOT antennae!

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