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I'm planning on putting a quarter wave ground plane antenna in my attic for use with 2m/70cm. In order to keep the feed line short, it needs to be near my TV antenna. Is there a rule of thumb for the minimum distance between the two antennas?

Accessing the attic is a real pain, so I'd like to put the new antenna in a good spot on the first try. The TV antenna is a yagi which is pointed away from area where I want to put the ham antenna. The ham antenna will be connected to a 5 watt transmitter initially, but I'd like to eventually upgrade to 50 watts.

Update

It's been a while and I'm not experiencing any interference issues. I went with the quarter wave ground plane antenna, even though it's omnidirectional. My radio transmits 5 watts. I installed the ham antenna about five feet behind the TV antenna, with the lower end of the radiating element about two feet higher than the TV antenna. The things that seem to be helping are:

  • The TV antenna is highly directional and points away from the ham antenna.
  • The antennas have different polarizations (horizontal for TV and vertical for ham). I suspect this is helping more than anything else.
  • The ham antenna doesn't radiate much power downward, so the vertical separation should be helping at least a bit.
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  • $\begingroup$ How much power at its input can your TV handle without damage? $\endgroup$ – Phil Frost - W8II Feb 2 '18 at 16:50
  • $\begingroup$ @PhilFrost-W8II I have no idea, and I'm pretty sure the manufacturer doesn't publish that data. Do you know what value is typical for TVs made in the last 20 years? $\endgroup$ – mrog Feb 2 '18 at 17:55
  • $\begingroup$ I've really no idea, but without that information it's pretty hard to objectively answer the question. $\endgroup$ – Phil Frost - W8II Feb 2 '18 at 18:40
  • $\begingroup$ @PhilFrost-W8II I'll settle for a subjective answer, as long as it's somewhat useful. $\endgroup$ – mrog Feb 2 '18 at 18:48
  • $\begingroup$ Recognizing that it's not possible to provide an exact answer, does anyone have any anecdotal evidence to offer? $\endgroup$ – mrog Feb 2 '18 at 18:50
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As other have commented, there is no sure way of avoiding damage or interference when the antennas are in close proximity. But there are a few things you can do to minimize the chance of interference.

Place the Ground Plane 180° From the Front of the TV Antenna

By placing the ground plane antenna in the opposite direction of the primary gain lobe of the TV antenna, the amount of signal from your VHF/UHF transmitter that is coupled into your TV antenna system will be minimized. The Friis equation helps to estimate the effect:

$$P_\text{rx}=P_\text{tx}G_\text{tx}G_\text{rx} \left(\frac{\lambda}{4\pi D}\right)^2 \tag 1$$

where Prx is the power in watts received by the receiver, Ptx is the power of the transmitter in watts, Gtx is the linear gain of the transmitting antenna, Grx is the linear gain of the receive antenna, $\lambda$ is the wavelength in meters, and D is the distance between antennas in meters. Note that the antennas cannot be in each other's near fields. This can be largely avoided by separating the antennas by at least 1 wavelength (2 meters).

To convert dB gain or loss to linear gain or loss:

$$G_\text{linear}=10^\frac{G_\text{dBi}}{10} \tag 2$$

If we assume that your ground plane antenna has a 2 dBi gain, the back end of the TV antenna has a -10 dBi gain, 5 watts of transmit power on two meters, and the distance between the antennas is 6 feet, formula 1 tells us that the power in the receive antenna will be 6.32 milliwatts. That is the equivalent of being within 1470 feet from a 15,000 watt ERP VHF television station with a 5 dBi gain TV antenna pointing directly at the TV station. That is a lot of power!

You can see from equation 1 that doubling the distance between the antennas, the received power is cut by 75%. Thus placing your antenna 12 feet from the TV antenna would result in 1.58 milliwatts at the antenna.

Avoid Using TV Antenna Amplifiers

TV antenna amplifiers are, by necessity, very wide bandwidth amplifiers. The power received from your ground plane antenna could cause IMD (intermodulation distortion) within the amplifier which could affect the desired TV signals.

Use Less Transmit Power

VHF and UHF stations primarily rely on line of site for communications. You may find that you can easily talk into a nearby repeater with a few 100 milliwatts. This will greatly reduce the possibility of interference with the TV system.

Use a Directional Transmit Antenna

A small VHF/UHF Yagi pointed away from the TV antenna could add 10 dB or more of path loss to the signal that gets coupled into the TV antenna. The practicality of this solution depends on the direction of the repeaters or other hams with which you wish to communicate.

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  • $\begingroup$ This looks like the best answer so far. It may not completely answer the question, but it's informative and useful. $\endgroup$ – mrog Feb 5 '18 at 23:01
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You may need to do some experiments. It will also depend on what band your TV channels are. You don't state where in the world you live. (TV channels can also be on multiple bands, e.g. in North America where VHF-HI and UHF are both still used in some areas).

My instinct is that greater separation will be your friend, especially when transmitting at higher power. This will also reduce the effects of your TV antenna on your signal pattern.

Alternatively, consider putting the TV antenna or the ham radio antenna outside, if your living arrangements permit it. Since you seem to be getting adequate service with an attic-mounted TV antenna, having the ham antenna outside would be ideal in terms of getting superior range.

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  • $\begingroup$ I should have thought to specify my location. I'm in the US. My TV antenna is connected to two receivers, both of which support both analog and digital signals. So, UHF and VHF are both a concern. I agree that distance is good, but distance is also hard (limited attic access), potentially expensive (feed line cost), and could cause trouble with the neighbors (HOA doesn't allow outside antennas). I'm not looking for an exact answer. Just some estimates or anecdotes would be a good starting point. $\endgroup$ – mrog Feb 2 '18 at 18:07
  • $\begingroup$ VHF frequencies for North American TV are just above the 2m band. UHF starts just over 70cm and extends to close to 23cm. So... very likely you'll get harmful interference. Band pass filters on your TV feed line would probably help immensely. $\endgroup$ – Jim MacKenzie VE5EV Feb 2 '18 at 18:15
  • $\begingroup$ I found that TVs here use three bands: VHF low (54-88 MHz), VHF high (174-216 MHz), and UHF (470-806 MHz). I've been looking at filter designs a bit. It should be fairly easy to block out the 2m band, but the 70cm band (420-450 MHz) is very close to the TV UHF band. I haven't found a design that achieves good separation between those bands. A 9-pole Chebyshev filter could help a bit, but it's still going to let in a lot of 70cm RF. Do you have any suggestions for a better kind of filter to use? $\endgroup$ – mrog Feb 5 '18 at 19:45
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    $\begingroup$ VHF-low has been de-deployed for television here in Canada, and a lot of regions in the US don't have any VHF stations at all, so it's worth checking into your local circumstances before you worry too much about your alternatives. Also, if the TV signals are strong enough, you may interfere, but because of how digital works, there may not be a harmful effect. $\endgroup$ – Jim MacKenzie VE5EV Feb 5 '18 at 20:02

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