To: | topband@contesting.com |
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Subject: | Re: Topband: Re: Increasing bandwidth 160m L |
From: | "Donald Chester" <k4kyv@hotmail.com> |
Date: | Fri, 31 Dec 2004 19:07:41 +0000 |
List-post: | <mailto:topband@contesting.com> |
Don't forget that sometimes increased bandwidth indicates losses within the
system. For example, a vertical with an inadequate ground system may
exhibit a large bandwidth, but as you install more radials the bandwidth
decreases as the efficiency increases. The ultimate example of the bandwidth vs efficiency tradeoff would be the infamous Maxcom antenna matcher, which was essentially a dummy load with nonresonant antenna wires attached. The wires radiated enough rf to produce a detectable signal at some distance, while the SWR remained close to 1:1 from below the broadcast band to vhf. I have seen other "broadband" antenna schemes that included resistive elements to broaden out the resonance characteristic. I believe B&W sold one sometime ago, that was reasonably (>50%) efficient. If you put up a new antenna and get unexpected bandwidth, treat in with suspicion until you have verified that there are no losses in the circuit that effectively add resistance. I built my quarter-wave vertical about 20 years ago. It consists of 127' of tower on a base insulator, with 120 buried quarterwave radials. Everything in the earthing system is brazed together with silver alloy solder. At 119' ( top guying level), I have the middle of a 135' long dipole attached, with open wire feeders going down to the base through the interior of the tower, using plexiglas spacers every 10'. When the tower is used as a vertical, the dipole feeders are disconnected at the tower base using a ceramic knife switch, and left floating. The base of the tower is matched to RG-213 using a simple L-network tuned for 1:1 SWR at 1900 khz. When I first fired it up, I had expected a bandwidth of about 60 kHz, based on previous experience with inverted-L's and shortened verticals. I was surprised that the bandwidth was broad enough that I could work the entire band, 1.8 - 2.0 mHz, with SWR not exceeding 2.5:1. The first thing I did was to check all connections for good electrical contact and make sure the ground radials and the rest of the earthing system had good integrity. Everything checked out OK, and when I started using the antenna I got the expected good signal reports throughout the band, all over N America. I just left the L-network permanently tuned to 1900 kHz and adjust the matching network between the transmitter and coax feedline when changing frequency. I have never tried to measure the field strength at a specific distance away from the tower to accurately determine its radiating efficiency. Base impedance measurements gave interesting results. Normally a quarter wave vertical can be expected to measure somewhere around 40 ohms. Mine measured as follows, using a General Radio 916-AL impedance bridge: (with the dipole feeders disconnected at the base and left floating) 1800 kHz 117 + j233 1900 kHz 185 + j316 2000 kHz 320 + j425 Thermocouple rf ammeter readings at known rf power levels verified the above measurements. Obviously, the dipole adds considerable top loading to the vertical, making it closer to the classic "vertical tee" than a true quarter wave vertical. The dipole is not electrically connected to the tower at any point, but the close proximity of the concentric feedline - tower combination provides the coupling. Interestingly, electrically connecting the ends of the disconnected open wire feeders to the tower base yielded the following results: 1800 kHz 28.8 - j10.5 1900 kHz 45.5 + j30.5 2000 kHz 70 + j99 I always use the vertical with the feedline floating at the base. I have never attempted to analyse the antenna, since the capacitive coupling of the tower to flat top through the feedline over the full length of the tower and feedline would be a very complex equation to calculate, much like the recently discussed topic of using a significant fraction of wavelength of coax as a capacitor. I happened upon an efficient wideband antenna system and have been taking advantage of it for over 20 years. -k4kyv
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