Re: Topband: Adding 80 m to 160 m quarterwave vertical
Hi Rob, I have a ceramic power pole insulator at the base of the tower, so I’m not concerned about voltage there. Incidentally, I measured the impedance of the tower at the base as 666 ohms at it’s resonant frequency of 3.14 MHz. At 3.51 MHz it is 391 – j386 ohms, so it’s longer than a half wavelength at 80 m. I’ll have to check that again in the morning, as I’d have thought it +j ohms. Ground system is 60 x 33.3 m insulated radials, buried. That should be about a half wavelength on 80 m. 73, Luke VK3HJ. Sent from Mail for Windows 10 From: Rob Atkinson Sent: Tuesday, 6 April 2021 9:15 PM To: topband@contesting.com Subject: Re: Topband: Adding 80 m to 160 m quarterwave vertical A ham can certainly try a voltage fed 180 degree vertical, but the physical conditions at the feedpoint change dramatically because it's quite possible a few KV of RF v. will be there, unlike what we have with a current maxima at the feedpoint. So insulation and keeping surfaces dry become much more important. You can't just use a block of wood exposed to rain for an insulator and if the base is on a concrete pier, it has to have copper strapped around it so the concrete isn't part of the v. gradient between the antenna and ground. Besides arcs, RF loss is a consideration, plus the ground system has to be extended because the induced earth currents are farther out. It's a lot of trouble and expense for a slightly lower take off. 73 Rob K5UJ _ Searchable Archives: http://www.contesting.com/_topband - Topband Reflector _ Searchable Archives: http://www.contesting.com/_topband - Topband Reflector
Re: Topband: Adding 80 m to 160 m quarterwave vertical
A ham can certainly try a voltage fed 180 degree vertical, but the physical conditions at the feedpoint change dramatically because it's quite possible a few KV of RF v. will be there, unlike what we have with a current maxima at the feedpoint. So insulation and keeping surfaces dry become much more important. You can't just use a block of wood exposed to rain for an insulator and if the base is on a concrete pier, it has to have copper strapped around it so the concrete isn't part of the v. gradient between the antenna and ground. Besides arcs, RF loss is a consideration, plus the ground system has to be extended because the induced earth currents are farther out. It's a lot of trouble and expense for a slightly lower take off. 73 Rob K5UJ _ Searchable Archives: http://www.contesting.com/_topband - Topband Reflector
Re: Topband: Adding 80 m to 160 m quarterwave vertical
On 4/5/2021 8:48 PM, Lloyd - N9LB wrote: Why not remove the 80m wire and simply run the entire 30m tall mast (with additional top hat) on 80m as a half-wave vertical? A "legal Limit" antenna tuner should handle it, or build your own 80m antenna matching unit. I did exactly what you describe with my 27m (90 ft) mast. It apparently worked fine (I didn't have another antenna to compare it to.) I built a homebrew tuner. I don't know of a commercial one that could handle the high voltages. I later decided this was too much trouble and installed a relay at the 60 foot level that allowed it to run as a conventional vertical on 80 when the relay was open. 73 Rick N6RK _ Searchable Archives: http://www.contesting.com/_topband - Topband Reflector
Re: Topband: Adding 80 m to 160 m quarterwave vertical
Why not remove the 80m wire and simply run the entire 30m tall mast (with additional top hat) on 80m as a half-wave vertical? A "legal Limit" antenna tuner should handle it, or build your own 80m antenna matching unit. -Original Message- From: Topband [mailto:topband-bounces+lloydberg=tds@contesting.com] On Behalf Of List Mail Sent: Monday, April 05, 2021 10:11 PM To: Topband Subject: Topband: Adding 80 m to 160 m quarterwave vertical I have up and operational a vertical for 160 m. It’s about 30 m of 250 mm triangular mast (Deeco) topped by about 5 m of 50 mm aluminium mast with a small top hat to resonate around 1840 kHz. Fed directly above the base insulator, it presents a reasonable match across the band, with about 32 ohms at resonance. I also want to operate 80 m DX. So far, I’ve been experimenting with a wire, offset by 1 metre, about 20.2 m long, parallel to the mast. I’ve tried grounding the 160 m mast and feeding the wire separately, floating the mast and feeding separately, various L-matches, and now feeding in parallel with the main mast with no matching. The latter gives me a reasonable match on the CW end of the band, and it doesn’t affect the 160 m operation. The complicating factor is the approximately halfwave antenna on 80 m in close proximity to the wire. Previously, I had a mast about 22 m high, fed through a series capacitor for 80 m, with an offset wire going up to a top hat at the top of the mast for 160 m. That seemed to work ok. Now I have the reverse situation. I made a couple of QSO on the 80 m DX window last week, with the offset wire about 19 m long, and it seemed to work ok. But my interest is not SSB DX. Adding lumped constants to try and match that wire on the CW end of the band didn’t work as expected, i.e., some base loading to bring down the resonant frequency, presumably due to the effect of the tower, grounded or floating. I lengthened the wire to resonate on the lower end of 80 m and made a few CW QSO, but either it’s performance or the propagation was disappointing. After all that, I’m getting a bit lost. What arrangements have been successfully used by others? Again, I have a near-enough to quarter wave base-insulated 160 m vertical, and want to operate also on 80 m CW, with a secondary priority of SSB DX Window. 73, Luke VK3HJ. Sent from Mail for Windows 10 _ Searchable Archives: http://www.contesting.com/_topband - Topband Reflector _ Searchable Archives: http://www.contesting.com/_topband - Topband Reflector
Topband: Adding 80 m to 160 m quarterwave vertical
I have up and operational a vertical for 160 m. It’s about 30 m of 250 mm triangular mast (Deeco) topped by about 5 m of 50 mm aluminium mast with a small top hat to resonate around 1840 kHz. Fed directly above the base insulator, it presents a reasonable match across the band, with about 32 ohms at resonance. I also want to operate 80 m DX. So far, I’ve been experimenting with a wire, offset by 1 metre, about 20.2 m long, parallel to the mast. I’ve tried grounding the 160 m mast and feeding the wire separately, floating the mast and feeding separately, various L-matches, and now feeding in parallel with the main mast with no matching. The latter gives me a reasonable match on the CW end of the band, and it doesn’t affect the 160 m operation. The complicating factor is the approximately halfwave antenna on 80 m in close proximity to the wire. Previously, I had a mast about 22 m high, fed through a series capacitor for 80 m, with an offset wire going up to a top hat at the top of the mast for 160 m. That seemed to work ok. Now I have the reverse situation. I made a couple of QSO on the 80 m DX window last week, with the offset wire about 19 m long, and it seemed to work ok. But my interest is not SSB DX. Adding lumped constants to try and match that wire on the CW end of the band didn’t work as expected, i.e., some base loading to bring down the resonant frequency, presumably due to the effect of the tower, grounded or floating. I lengthened the wire to resonate on the lower end of 80 m and made a few CW QSO, but either it’s performance or the propagation was disappointing. After all that, I’m getting a bit lost. What arrangements have been successfully used by others? Again, I have a near-enough to quarter wave base-insulated 160 m vertical, and want to operate also on 80 m CW, with a secondary priority of SSB DX Window. 73, Luke VK3HJ. Sent from Mail for Windows 10 _ Searchable Archives: http://www.contesting.com/_topband - Topband Reflector
