Re: Topband: 160 Tower on 80
This is why wide broadcast towers, even 1/2 wave tall towers, can have reasonably low impedances at the base.² Agreed. I have found it very difficult to model accurate complex base Z measurements of wide broadcast towers (i.e., low height/diam. ratio) that are of the 180-195 degree variety - even with NEC4.2. This is one of NEC¹s more significant limitations. In some of these cases, lighting systems and any STL transmission lines play a part in the true base Z. Still, I routinely get results that show much higher base Z in the model than actual measured base Z results. Paul, W9AC _ Topband Reflector Archives - http://www.contesting.com/_topband
Re: Topband: 160 Tower on 80
Hi all, I have a 90 foot Rohn 25 tower with an insulated base and insulated guy wire sections for top loading ,base fed for 160 meters. It work great ,but I would like to use it on 80 meters as well. The 3 ideas I have considered are voltage feed at the base with a resonant LC network at the base, but I am a little worried about the voltages present at legal limit power. Second idea,disconnect the top loading and put a trap between the top loading and the tower to divorce the top loading on eighty then an L network at the base for 80. Third idea, run a wire as a sloper either a quarter wave fed against ground or a 1/2 wave dipole from the tower. Any thoughts or alternative ideas would be greatly appreciated. Hi Glen, Every antenna is also a transmission line. Every conductor making up an antenna has a surge impedance. That surge impedance, along with several other factors, determines the base voltage. If the conductor is uniform size, lossless, not coupled very well to space, and infinitely thin, the voltage at some points along the length can extremely high. As it is made thicker or loss is added, voltage greatly decreases no matter what the length. This is why wide broadcast towers, even 1/2 wave tall towers, can have reasonably low impedances at the base. A 90 foot tower against lossless perfect ground, with hat wires to make it resonant on 160 meters, has about 2kV peak base voltage at 1500 watts on 80 meters. The base impedance is only around 1000-1500 ohms. This is no worse than voltages typically encountered in traps of trap Yagi antennas, so it isn't astronomical. It won't be anything at all like feeding a vertical #38 AWG wire for voltage or impedance. :) 73 Tom _ Topband Reflector Archives - http://www.contesting.com/_topband
Re: Topband: 160 Tower on 80
Any thoughts or alternative ideas would be greatly appreciated. Glenn WW4B Hi Glenn, all of Charlie's, K4OTV ideas work of course but you did mention alternate ideas. I also have a Rohn 25 insulated base tower that is 142 feet to the top of the stinger that I use for 160. By far and the easiest solution 100% guaranteed to work for a clean switch from 80 to 160 is a second tower some distance away. Used 25 is relatively cheap and Philly now is sold by a lot of folks. The only switching is in the shack, etc. Simple is good. I chose not to use the 2nd tower as a vertical and it supports a 4 square using the DXE hybrid. For the radiating elements I used dipoles with shield end sharply folded back to the tower. Because the elements come pretty close together at the top you do lose a little gain but not that much over verticals and verticals require a lot of radials where this array just has the one element folded back for each direction. This array seems to work so well I build another for 40 meters under a full size 40 Yagi. At times the Yagi is better as expected but the ops here like the instant switching and use it a lot. Added bonus these arrays survive the winter storms here with 100 mph + winds. 73 Rich KL7RA (now on topband but no one can hear me yet.) Well, Glenn, I would lean toward voltage-feed at the base, since you have that insulated tower! Because of the lower current, the ground-losses would be minimized, and the current maximum would be fairly high on the tower, resulting in a nice low-elevation angle, omni-directional radiator for long haul 80m DX work. Of course you would need a robust tapped parallel LC network with a robust inductor, probably mad of copper tubing and a really high voltage air variable capacitor, or better, a vacuum variable and there would be some substantial switching challenges to switch the feed between 80 and 160, that would be avoided with your trapped top loading approach, but full-power 80m traps 90' in the air are non-trivial components also! I LIKE the voltage-fed 1/2 wave vertical approach and have had wonderful success with it on 40m, where I fed the vertical 1/2 wave through a 1/4 wave 450 om line for an almost perfect match!! Great antenna. And I used it as the driver for a wide-spaced 3-element vertical 40m yagi for Peter 1 Island and Bouvet and it was a killer antenna for SE Asia on the evening LP. So, that's my $0.02! If it were me, I'd leave the top-loading alone and do the work to voltage feed that insulated tower! Should be a great antenna for both 80 and 160, and has the advantage that all the tinkering and tweaking can be done at ground level! GL and have fun!! 73, Charlie, K4OTV -Original Message- From: Topband [mailto:topband-boun...@contesting.com] On Behalf Of Glenn Biggerstaff Sent: Saturday, February 28, 2015 8:16 PM To: topband@contesting.com Subject: Topband: 160 Tower on 80 Hi all, I have a 90 foot Rohn 25 tower with an insulated base and insulated guy wire sections for top loading ,base fed for 160 meters. It work great ,but I would like to use it on 80 meters as well. The 3 ideas I have considered are voltage feed at the base with a resonant LC network at the base, but I am a little worried about the voltages present at legal limit power. Second idea,disconnect the top loading and put a trap between the top loading and the tower to divorce the top loading on eighty then an L network at the base for 80. Third idea, run a wire as a sloper either a quarter wave fed against ground or a 1/2 wave dipole from the tower. Any thoughts or alternative ideas would be greatly appreciated. Glenn WW4B _ Topband Reflector Archives - http://www.contesting.com/_topband _ Topband Reflector Archives - http://www.contesting.com/_topband _ Topband Reflector Archives - http://www.contesting.com/_topband
Re: Topband: 160 Tower on 80
Hi, Glenn There's a lot to be said for Rich's approach if you can manage the additional tower. It does need to be well separated from the 160 tower as some interaction is likely. You might want to do some modeling in EZNEC before installing the additional tower. Still phased, or parasite arrays can provide substantial results for 80 and 160! I built a large 5- element steerable array for a friend for 80 meters. It had a central 1/4 wave GP radiartor and 4 surrounding GPs that could be switched between reflector and director tuning with stubs of ladder line that could be remotely shorted with relays to achieve different directions. It was a KILLER antenna for 80 and 75 meters and Jim didn't wait fr anything!! - Even to breaking EU pile-ups for A61 with ONE CALL from Virginia!! It was a HUGE success! So, some variatiom of Rich's approach might well have merit! I would strongly recommend some advance modeling and experimentation with EZNEC before installing the second tower. I have thought of putting an 80m trap in the top horizontal wire for my inverted L, but I didn't like the idea of an 80 m trap 70' in the air. So I elected to add a 2nd vertical radiator for 80m and feed it in parallel with the 160 inverted L. That's also an option for you, if you add an 80m radiator in parallel and run it up the side of your tower. If you are using elevated radials, you[ would need to add an additional set for 80m. Anyway, GL and have fun! 73, Charlie, K4OTV -Original Message- From: KL7RA [mailto:kl...@ptialaska.net] Sent: Saturday, February 28, 2015 10:53 PM To: Charlie Cunningham; 'Glenn Biggerstaff'; topband@contesting.com Subject: Re: Topband: 160 Tower on 80 Any thoughts or alternative ideas would be greatly appreciated. Glenn WW4B Hi Glenn, all of Charlie's, K4OTV ideas work of course but you did mention alternate ideas. I also have a Rohn 25 insulated base tower that is 142 feet to the top of the stinger that I use for 160. By far and the easiest solution 100% guaranteed to work for a clean switch from 80 to 160 is a second tower some distance away. Used 25 is relatively cheap and Philly now is sold by a lot of folks. The only switching is in the shack, etc. Simple is good. I chose not to use the 2nd tower as a vertical and it supports a 4 square using the DXE hybrid. For the radiating elements I used dipoles with shield end sharply folded back to the tower. Because the elements come pretty close together at the top you do lose a little gain but not that much over verticals and verticals require a lot of radials where this array just has the one element folded back for each direction. This array seems to work so well I build another for 40 meters under a full size 40 Yagi. At times the Yagi is better as expected but the ops here like the instant switching and use it a lot. Added bonus these arrays survive the winter storms here with 100 mph + winds. 73 Rich KL7RA (now on topband but no one can hear me yet.) Well, Glenn, I would lean toward voltage-feed at the base, since you have that insulated tower! Because of the lower current, the ground-losses would be minimized, and the current maximum would be fairly high on the tower, resulting in a nice low-elevation angle, omni-directional radiator for long haul 80m DX work. Of course you would need a robust tapped parallel LC network with a robust inductor, probably mad of copper tubing and a really high voltage air variable capacitor, or better, a vacuum variable and there would be some substantial switching challenges to switch the feed between 80 and 160, that would be avoided with your trapped top loading approach, but full-power 80m traps 90' in the air are non-trivial components also! I LIKE the voltage-fed 1/2 wave vertical approach and have had wonderful success with it on 40m, where I fed the vertical 1/2 wave through a 1/4 wave 450 om line for an almost perfect match!! Great antenna. And I used it as the driver for a wide-spaced 3-element vertical 40m yagi for Peter 1 Island and Bouvet and it was a killer antenna for SE Asia on the evening LP. So, that's my $0.02! If it were me, I'd leave the top-loading alone and do the work to voltage feed that insulated tower! Should be a great antenna for both 80 and 160, and has the advantage that all the tinkering and tweaking can be done at ground level! GL and have fun!! 73, Charlie, K4OTV -Original Message- From: Topband [mailto:topband-boun...@contesting.com] On Behalf Of Glenn Biggerstaff Sent: Saturday, February 28, 2015 8:16 PM To: topband@contesting.com Subject: Topband: 160 Tower on 80 Hi all, I have a 90 foot Rohn 25 tower with an insulated base and insulated guy wire sections for top loading ,base fed for 160 meters. It work great ,but I would like to use it on 80 meters as well. The 3 ideas I have considered are voltage feed at the base with a resonant LC network at the base
Re: Topband: 160 Tower on 80
Hi Glenn, You should be very worried about voltage breakdown of your base insulator! Do you know how much RF voltage your base insulator can safely handle? A fourth idea you might consider is to simply use your tower to support a 1/4 wave vertical wire for 80 meters, fed against the same radials you use on 160 meters. This may be your easiest and most effective solution. I'm sure other Topbanders use the same solution for 80 meters. While a 1/2 wave vertical theoretically has a small amount of gain compared to a 1/4 wavelength vertical, in actual practice useful gain cannot be achieved when its mounted very close to average soil. The situation would be different in a salt marsh. 73 Frank W3LPL -Original Message- From: Topband [mailto:topband-boun...@contesting.com] On Behalf Of Glenn Biggerstaff Sent: Saturday, February 28, 2015 8:16 PM To: topband@contesting.com Subject: Topband: 160 Tower on 80 Hi all, I have a 90 foot Rohn 25 tower with an insulated base and insulated guy wire sections for top loading ,base fed for 160 meters. It work great ,but I would like to use it on 80 meters as well. The 3 ideas I have considered are voltage feed at the base with a resonant LC network at the base, but I am a little worried about the voltages present at legal limit power. Second idea,disconnect the top loading and put a trap between the top loading and the tower to divorce the top loading on eighty then an L network at the base for 80. Third idea, run a wire as a sloper either a quarter wave fed against ground or a 1/2 wave dipole from the tower. Any thoughts or alternative ideas would be greatly appreciated. Glenn WW4B _ Topband Reflector Archives - http://www.contesting.com/_topband _ Topband Reflector Archives - http://www.contesting.com/_topband _ Topband Reflector Archives - http://www.contesting.com/_topband
Re: Topband: 160 Tower on 80
In this case a three or four wire cage with 36 inch spreaders cut for 80 meters might be the easiest solution for you and allow for feeding at ground level. However the impact of the tower might result in requiring the cage to be a bit shorter than normal. Herb Schoenbohm, KV4FZ On 3/1/2015 3:16 AM, donov...@starpower.net wrote: A fourth idea you might consider is to simply use your tower to support a 1/4 wave vertical wire for 80 meters, fed against the same radials you use on 160 meters. This may be your easiest and most effective solution. _ Topband Reflector Archives - http://www.contesting.com/_topband
Re: Topband: 160 Tower on 80
On 2/28/2015 5:15 PM, Glenn Biggerstaff wrote: Hi all, I have a 90 foot Rohn 25 tower with an insulated base and insulated guy wire sections for top loading ,base fed for 160 meters. It work great ,but I would like to use it on 80 meters as well. The 3 ideas I have considered are voltage feed at the base with a resonant LC network at the base, but I am a little worried about the voltages present at legal limit power. Second idea,disconnect the top loading and put a trap between the top loading and the tower to divorce the top loading on eighty then an L network at the base for 80. Third idea, run a wire as a sloper either a quarter wave fed against ground or a 1/2 wave dipole from the tower. Any thoughts or alternative ideas would be greatly appreciated. Glenn WW4B _ I have a 90 foot top loaded vertical made from irrigation pipe instead of R25. Should be similar electrically. I have done the voltage feed method on 80 with it, and it works perfectly. You just have to have the HV components and be comfortable with designing a simple matching network. If you were using tuned elevated radials on 160, this would make sense because it would relieve you of having to have a set of 80 meter radials. If you have normal ground radials like me, you might want to consider a relay to switch out the top loading wires. I currently have an insulated joint at the 60 foot point on my vertical with a relay across it. When the relay is open, I have a 1/4 wave 80 meter vertical. In your case, you would have a 3/8 wave 80 meter vertical, that you could tune to resonance with a series capacitor. For details on the relay, see my HK article: May 2009 - QST (Pg. 66) Increasing Relay Voltage Handling Capacity You should put a string of 10 meg resistors across the relay to drain static charge. Rick N6RK _ Topband Reflector Archives - http://www.contesting.com/_topband
Re: Topband: 160 Tower on 80
Glenn, If the tower as is, with the top loading on 160 causing it to be long for 80 meters for an easy feed, you can make a simple decoupling sleeve skirt and feed the tower with a gamma rod on 80 meters so the RF then sees a shorter tower. There might be some impact on the 160 meter feed configuration but most likely that would only some retuning of the 160 match. But apart from that the separate 80 meter gamma rod above the decoupling sleeve and finding the sweet point without the need for a trap or a dedicated L/C circuit at the base of a tower that appears to long, is one possible solution. In summary just shorten the tower electrically on 80 by a simple decoupling skirt of 4 wires coming down the tower and flaring out at the base as you may require. A fixed or variable capacitor between the coax feed and the gamma rod to remove the inductive reluctance of the rod should be all that is needed. In essence you have then created a coaxial vertical dipole. At the point where the 80 meter feed line leaves the tower near the base, I think most would recommend isolating the feed line with a ferrite ring. I also remember Earl Cunningham K6SE, (SK) did an article on a variation of this some time ago and he seemed pleased with the performance on 80 meters. Earl, I understand, did not employ a skirt but just put an 80 meter gamma rod on the tower used on 160. I think however Earl's tower was a bit shorter and yours as described most likely approaches 1/2 wave on 80 meters. A decoupling 4 wire sleeve changes all that. Herb Schoenbohm, KV4FZ On 2/28/2015 9:15 PM, Glenn Biggerstaff wrote: Hi all, I have a 90 foot Rohn 25 tower with an insulated base and insulated guy wire sections for top loading ,base fed for 160 meters. It work great ,but I would like to use it on 80 meters as well. The 3 ideas I have considered are voltage feed at the base with a resonant LC network at the base, but I am a little worried about the voltages present at legal limit power. Second idea,disconnect the top loading and put a trap between the top loading and the tower to divorce the top loading on eighty then an L network at the base for 80. Third idea, run a wire as a sloper either a quarter wave fed against ground or a 1/2 wave dipole from the tower. Any thoughts or alternative ideas would be greatly appreciated. Glenn WW4B _ Topband Reflector Archives - http://www.contesting.com/_topband _ Topband Reflector Archives - http://www.contesting.com/_topband
Re: Topband: 160 Tower on 80
Well, Glenn, I would lean toward voltage-feed at the base, since you have that insulated tower! Because of the lower current, the ground-losses would be minimized, and the current maximum would be fairly high on the tower, resulting in a nice low-elevation angle, omni-directional radiator for long haul 80m DX work. Of course you would need a robust tapped parallel LC network with a robust inductor, probably mad of copper tubing and a really high voltage air variable capacitor, or better, a vacuum variable and there would be some substantial switching challenges to switch the feed between 80 and 160, that would be avoided with your trapped top loading approach, but full-power 80m traps 90' in the air are non-trivial components also! I LIKE the voltage-fed 1/2 wave vertical approach and have had wonderful success with it on 40m, where I fed the vertical 1/2 wave through a 1/4 wave 450 om line for an almost perfect match!! Great antenna. And I used it as the driver for a wide-spaced 3-element vertical 40m yagi for Peter 1 Island and Bouvet and it was a killer antenna for SE Asia on the evening LP. So, that's my $0.02! If it were me, I'd leave the top-loading alone and do the work to voltage feed that insulated tower! Should be a great antenna for both 80 and 160, and has the advantage that all the tinkering and tweaking can be done at ground level! GL and have fun!! 73, Charlie, K4OTV -Original Message- From: Topband [mailto:topband-boun...@contesting.com] On Behalf Of Glenn Biggerstaff Sent: Saturday, February 28, 2015 8:16 PM To: topband@contesting.com Subject: Topband: 160 Tower on 80 Hi all, I have a 90 foot Rohn 25 tower with an insulated base and insulated guy wire sections for top loading ,base fed for 160 meters. It work great ,but I would like to use it on 80 meters as well. The 3 ideas I have considered are voltage feed at the base with a resonant LC network at the base, but I am a little worried about the voltages present at legal limit power. Second idea,disconnect the top loading and put a trap between the top loading and the tower to divorce the top loading on eighty then an L network at the base for 80. Third idea, run a wire as a sloper either a quarter wave fed against ground or a 1/2 wave dipole from the tower. Any thoughts or alternative ideas would be greatly appreciated. Glenn WW4B _ Topband Reflector Archives - http://www.contesting.com/_topband _ Topband Reflector Archives - http://www.contesting.com/_topband
