HFU HF Underground
Technical Topics => Equipment => Topic started by: Matt285 on February 25, 2020, 2201 UTC
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So I got my Discone installed to use with my RSP2 SDR receiver. Weather stations, Air traffic control etc work quite well, but I'm a bit disappointed with the transmit performance on 2m/440. The antenna is isolated from being grounded currently due to having to use electrical tape to build up the mast enough to get a fit. I plan on installing a jumper to ground it to the mast when I can. Set up is a Tram 1411 with approx 25' of 1/2' hard liine. Will the grounding provide better performance perhaps? Thanks
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How are you measuring its performance? Ideally you need to hook it up to an analyzer -- that would at least tell you how resonant the discone is at those frequencies. Discone antennas have little if any gain to them and are omni-directional. You won't "get out" like you would a resonant vertical antenna.
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Wide B/W at the expense of zero gain.
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While many discones are advertised as being able to be used (in transmit mode) on 2m/440, performance is generally quite poor. If you want a simple antenna for either of those bands, consider a J Pole.
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I've used these dual band ground planes in several projects. They hold up very well in the weather.
http://www.arrowantennas.com/gp/gp146.html
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The VHF +discone antennas+ are made for short distance coverage. They aim all around, but below the horizon, and should be installed on the higher place to see all the surroundings.
For longer distances, use the groundplane antenna. Or a "double discone" (conical vertical dipole).
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but I'm a bit disappointed with the transmit performance on 2m/440.
A discone is a compromise antenna. That does not mean it is bad, only that it trades off some aspects of performance to gain in other aspects. Specifically, to gain bandwidth the Discone accepts some less than optimal factors in other areas. There ain't no such thing as a free lunch. The Tram 1411 pushes this further than most.
A Discone can be quite broadbanded, but it often does not have really good SWR anywhere, in fact for large sections of its range it can be ~2:1. For receive applications this does not matter much at all, but we could end up in quite the discussion, with differing opinions, on how much this does to transmit applications. Suffice to say, a good monoband or dual band 2 meter or / and 70 cm antenna will typically have much better SWR and transmit performance.
Notice the vertical element on the Tram 1411 above the disc? That is there to extend the frequency range of the antenna, increasing the bandwidth further on the low end. However it also tends to have the affect of reducing the efficiency of the antenna. Excluding that vertical element, and for a simple discone, the size of the disc and cone, and the ratio of the cone to the disc, determines the lower frequency limit (roughly, the cone length should be 1/4 wavelength at the lowest intended frequency, the disc should have a diameter of 0.7 that length), the upper frequency limit is determined by a combination of the insulator gap between the disc and cone and the angle of the cone at that insulator. In the case of the Tram 1411 the situation is made worse by the lower adjustable radials. These are there to further enhance transmit bandwidth on the lower frequency end, at the expense of "normal" discone operation.
The gain of a discone is low, it is often quoted as about 0 dBi, but it actually varies quite a bit across its bandwidth. It might be best said it averages about 0 dBi, with some ranges having slightly higher gain and many areas significantly less, this last part is especially true if we are looking at usable gain vs maximum gain, as in how the pattern is shaped and how that impacts performance.
The radiation pattern of the Discone can be a problem. At the lower frequency end it can be pretty smooth and predictable, illuminating the horizon reasonably well. As the frequency goes up the pattern becomes more peaky, meaning the best lobe of the pattern may not be pointed at the horizon. You can end up with a main beam pointing down slightly, and another beam or three pointing up. The gross gain between those beams may be roughly 0 dBi, but the energy is split between them.
A purpose built 2m / 70 cm antenna will typically have significantly higher gain than a discone. More importantly, the main beam is generally better placed and shaped to illuminate the horizon.
I use several discones here. However I don't typically use them for transmit except for local, as in here in the yard, stuff. For anything other than the yard I use band specific antennas. The discone is really quite good as a jack of all trades antenna, but it is the master of none. In the military and in industry the discone is typically used where the wide bandwidth is required but the transmit performance either does not matter or can be overcome by other factors, such as increased transmitter power.
The antenna is isolated from being grounded currently due to having to use electrical tape to build up the mast enough to get a fit. I plan on installing a jumper to ground it to the mast when I can. Set up is a Tram 1411 with approx 25' of 1/2' hard liine. Will the grounding provide better performance perhaps? Thanks
Grounding the antenna will probably make no difference in performance. With that said, you should ground the antenna. While it may not improve performance as in change the radiation pattern or gain significantly, it may reduce any potential noise issues and increased safety.
T!
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A disconey is more or less a high pass filter at the end of a coax, you can find the formulae for rolling your own, or determining where yours is operating, in the antennae encomiums. A stinger on top (hopefully) extends the range to below what would otherwise be the cutoff point, and perhaps exhibits some gain for the bands the stinger covers. An low noise amp would be a very good idea for a discone.
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An low noise amp would be a very good idea for a discone.
Hehe this depends on where you live. High RF area == forget about it. I use (3) filters just to keep my signals in line.
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Good point, a filter for fmbc and/or paging might be needed in certain instances - usually a large city or anywhere near a hospital. I use a 50MHz highpass and a fmbc trap inline with mine.
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I use a 50MHz highpass and a fmbc trap inline with mine.
Yep, I use a 133 Mhz highpass (M/C), NOAA notch (PAR) and a RTL-SDR FM notch.