Re: Topband: Where to place a preamp? Switching Beverages?
On Fri, 2012-01-20 at 10:53 -0500, Pete Smith N4ZR wrote: Having totally struck out raising this topic on Towertalk, I'm bringing it here in the hopes of getting some solid advice. I have a 20 dB ARR preamp of the type praised by ON4UN in his book. I want to use it with a variety of RX antennas - loops, BOGs, etc. - that have very low output levels. My choice is whether to put it at the antenna end, incurring the added complexity of sending 12V DC to it via the coax, or to put it in the shack. I realize that if this was 2 GHz, there'd be no question where it should go. But for 160-meter purposes, I don't know. I am wondering if putting it at the antenna end might enhance directivity, by amplifying the signals from the directive antenna relative to any pickup on the 350-foot feedline. I will be trying to choke as much common mode current as possible, at both ends, but am not sure how effective I can be. On another topic, I am building a hub for BOGs. Rather than switch at 75 ohms and putting a matching transformer on each BOG, I wonder if it would be feasible/desirable to switch at the high-impedance side and use a single transformer. It would certainly simplify the hub. Any advice would be much appreciated. Hi Pete, I'm thinking if your 350 foot feedline is coax and your antenna output is very low it will be even lower at the shack end before it meets the preamp. I think I would put mine at the antenna end of the feedline and for the same reasons as you would at microwave frequencies. Once the received signal is 'lost' no amplifier is going to get it back. 73, Bill KU8H ___ UR RST IS ... ... ..9 QSB QSB - hw? BK
Re: Topband: Where to place a preamp? Switching Beverages?
Hi Pete, The common mode issue doesn't really affect your decision. You will need to ensure that common mode pickup on the coax doesn't get into the antenna, which ever position you choose for your preamp. This is really important for low output antennas like the Pennant The decision on whether to place it at the Rx and or the antenna end, depends on the expected signal level at the antenna, the loss in the coax, and the sensitivity of your receiver. These are exactly the same parameters as you would use to make the decision at UHF and above. Here is an example. Typical band noise output from a small Rx antenna in a quiet location on 160m might be -127 dBm in a 500 Hz BW. A typical noise figure for an HF preamp might be 6 dB, putting the noise floor at -168 dBm/Hz, or -141 dBm in 500 Hz. With no additional coax loss, your Rx will degrade the available S/N by 0.1 dB. Now with 350 ft of good quality RG6, you can expect around 2 dB loss, dropping your antenna noise down to -129 dBm, still 12 dB above the Rx noise floor. The Rx now degrades the available S/N by 0.3 dB. Provided your coax loss is not much worse than this, I wouldn't worry about it and put the preamp where it's convenient for you, but I advise you to do the maths and insert the actual figures for your own situation. 73, Greg, ZL3IX My choice is whether to put it at the antenna end, incurring the added complexity of sending 12V DC to it via the coax, or to put it in the shack. ___ UR RST IS ... ... ..9 QSB QSB - hw? BK
Re: Topband: Where to place a preamp? Switching Beverages?
N4ZR said : I have a 20 dB ARR preamp, My choice is whether to put it at the antenna end, incurring the added complexity of sending 12V DC to it via the coax, or to put it in the shack. Per the ON4UN book, In most cases you can put the preamplifier in the shack. The signal loss in the feed line is a loss that affects both the signal and external noise. That means that the loss in the feedline does not affect the S/N ratio. I personally have 3 point fed pennants that are very small (51.6% the size of full size pennants), and therefore their gain is around -46dbi, and my preamp is located in the shack (W1FB slightly modified preamp). I've done some simple tests with my preamp out at the feedpoint versus in the shack and I personally was unable to detect any difference in S/N performance but my measurement system was not highly sophisticated. My feedline is 160 feet of RG58/U, and I intentionally have no breaks (no connectors) in this feedline (one solid run of feedline from the connector on the back of my preamp to the primary of my transformer which is located at the antennas (my feedline is soldered directly to the transformer primary), and I did this to eliminate any and all weather related connector problems. I only use one transformer to feed my 3 pennants, and I do switch the high impedance side of the transformer (the transformer secondary), and I switch both ends of the secondary (mandatory for multiple point fed pennant systems). I have 14 turns of my coax run through 3 stacked 2.4 O.D. 31 mix cores to help prevent common mode current, and this choke is located approximately 25 feet away from the base of multiple pennant feedpoint. Also have 14 turns of my relay control cable (CAT 5 cable) running through 2 stacked 2.4 O.D. 31 mix cores which are located on the ground below my antenna feedpoint. 73's Don (wd8dsb) ___ UR RST IS ... ... ..9 QSB QSB - hw? BK
Re: Topband: Where to place a preamp? Switching Beverages?
I feed my normal Beverages with 320' of RG-58. and sometimes (at other locations) feed a Beverage with a shorter cable. Never ever have I been able to pull the antenna from the transformer between antenna and coax and have quietness in the receiver. Always there is too much pickup from AM BC stations (which I DX) and also noise. So I'll agree with Guy that few people's coax feeds are as clean as they think. For that reason, I'd put the preamp at the transformer so that the receiver thinks there's more S in the numerator with the same N in the denominator. Of course in a low noise environment, there will be little or no difference. The loss is immaterial to me. At .4 dB/100 ft at 1 MHz, 300 feet of RG-58 has 1.4 dB loss on paper and measured results show somewhat less loss. Chuck On 1/20/2012 10:44 AM, Guy Olinger K2AV wrote: I am getting comments from people who have installed FCP plus isolation transformer 160 TX antennas, how their new TX antenna is quieter than their K9AY or pennant, maybe a third or half of respondents (!!!) with some hint or outright statement of this. But I think, rather than the TX antenna being all that good, it's really the RX antenna's common mode isolation really being that grotesquely BAD. I can make this happen in a model by putting the coax on the ground and making the now low velocity factor ELECTRICAL length of the coax on the ground somewhat near a multiple of a halfwave. Typically something in the range of 125 or 150 feet, give or take, can have this VOLTAGE node, high Z point at one or both of the typical common mode blocking points. If one models this literally, and puts an EZNEC source on the shield, then you need common mode blocks in the 100k+ order of magnitude to keep noise down low enough to protect a non-amplified pennant antenna. The reason the TX FCP + isolation transformer is so quiet is that the UNCONNECTED windings of the transformer have only the capacitance between windings as a through path, and at the low 160m frequency the isolation is in the half megohm range against a 75 ohm-ish shunt to ground at amp or RX. So from where I'm setting, transformer ISOLATED preamps at EACH RX antenna need to be PROVED OUT for naturally lossy RX antennas, rather than the other way around, and you may need to run a separate DC lead to the preamp. I'm not saying they CAN'T be proved out, it's just that the beginning assumption needs to be that they are needed UNTIL it's proven they aren't need. I think that what is amiss is our perceptions about how quiet we think coax shields are. Apparently coax shields are just plain gawd-awful noisy, and must be assumed to be just plain gawd-awful noisy in the planning and construction stages. And I still get correspondence where the writer thinks they can evaluate RX antenna performance by A/B tests and how loud the desired signal is. Signal to noise cannot be done with the human ear and an A/B switch. Nobody's ear is that good. Borrow the equipment, do the work, get it right. Good 160 RX is a nasty, technical, unforgiving business with a mean mind toward deception. 73, Guy. On Fri, Jan 20, 2012 at 12:08 PM, Don Kirkwd8...@aol.com wrote: N4ZR said : I have a 20 dB ARR preamp, My choice is whether to put it at the antenna end, incurring the added complexity of sending 12V DC to it via the coax, or to put it in the shack. Per the ON4UN book, In most cases you can put the preamplifier in the shack. The signal loss in the feed line is a loss that affects both the signal and external noise. That means that the loss in the feedline does not affect the S/N ratio. I personally have 3 point fed pennants that are very small (51.6% the size of full size pennants), and therefore their gain is around -46dbi, and my preamp is located in the shack (W1FB slightly modified preamp). I've done some simple tests with my preamp out at the feedpoint versus in the shack and I personally was unable to detect any difference in S/N performance but my measurement system was not highly sophisticated. My feedline is 160 feet of RG58/U, and I intentionally have no breaks (no connectors) in this feedline (one solid run of feedline from the connector on the back of my preamp to the primary of my transformer which is located at the antennas (my feedline is soldered directly to the transformer primary), and I did this to eliminate any and all weather related connector problems. I only use one transformer to feed my 3 pennants, and I do switch the high impedance side of the transformer (the transformer secondary), and I switch both ends of the secondary (mandatory for multiple point fed pennant systems). I have 14 turns of my coax run through 3 stacked 2.4 O.D. 31 mix cores to help prevent common mode current, and this choke is located approximately 25 feet away from the base of multiple pennant feedpoint. Also
Re: Topband: Where to place a preamp? Switching Beverages?
- Original Message - From: Don Kirk wd8...@aol.com To: n...@contesting.com; Topband@contesting.com Sent: Friday, January 20, 2012 12:08 PM Subject: Re: Topband: Where to place a preamp? Switching Beverages? N4ZR said : I have a 20 dB ARR preamp, My choice is whether to put it at the antenna end, incurring the added complexity of sending 12V DC to it via the coax, or to put it in the shack. Per the ON4UN book, In most cases you can put the preamplifier in the shack. The signal loss in the feed line is a loss that affects both the signal and external noise. That means that the loss in the feedline does not affect the S/N ratio. That is a very misleading statement typical of all revisions and corrections to that book. Any loss before the first active device will degrade the MDS, noise figure, SNR, or whatever else you wish to call it. A low NF preamp with sufficient gain will overcome the feedline loss as well as the radios front end noise resulting in a better SYSTEM noise figure, MDS, SNR, etc. While the 350' feed of the OP should have a minimal loss of about .8dB at 2 MHz and an in shack preamp would be sufficient 99.9% of the time others with more loss should do the calculations to determine the cost/benefit of both settings. Carl KM1H ___ UR RST IS ... ... ..9 QSB QSB - hw? BK
Re: Topband: Where to place a preamp? Switching Beverages?
Guy Olinger K2AV wrote: I am getting comments from people who have installed FCP plus isolation transformer 160 TX antennas, how their new TX antenna is quieter than their K9AY or pennant, maybe a third or half of respondents (!!!) with some hint or outright statement of this. But I think, rather than the TX antenna being all that good, it's really the RX antenna's common mode isolation really being that grotesquely BAD. This advice is spot on and something I have advocated for years. Transformer coupling is the only way to go for feedline isolation. Unfortunately, many hams want to use a common mode choke because they are comfortable with that concept. The DX engineering active antenna makes this mistake, and as a result has poor feedline isolation which made it unusable in my application. Now the argument for common mode chokes and against transformers is that it it easy to send DC through the former. The correct solution is to send the RF through a transformer and send the DC through a small inductor that is self resonant at the frequency of interest. This permits the preamp to be transformer isolated from the feedline. Note that this technical problem is independent of whether you send DC over the coax or over a separate wire. The separate wire still requires the self resonant inductor to isolate it from the antenna side of the transformer. If you are operating on more than one band, get a resonant inductor for each band and put them in series. They will interact, but not at frequencies in the ham bands. Finally, my experience is that a vertical is the worst possible receive antenna, even if transformer isolated (yep, tried that). If your receive antenna is not as good as a vertical, it has serious problems. Rick N6RK ___ UR RST IS ... ... ..9 QSB QSB - hw? BK
Re: Topband: Where to place a preamp? Switching Beverages?
Thanks, everyone, for too many good inputs to list here. A lot of good ideas, both for experimental approaches and for the final configuration. At least for experimental purposes, I will go with the preamp in the shack and, assuming that my feedline is not quiet, significant improvements in management of common mode currents. One correspondent advocated transformer coupling instead of common mode chokes, and I'll need to correspond further with him on that, because I don't understand quite how it would work. 73, Pete N4ZR The World Contest Station Database, updated daily at www.conteststations.com The Reverse Beacon Network at http://reversebeacon.net, blog at reversebeacon.blogspot.com, spots at telnet.reversebeacon.net, port 7000 and arcluster.reversebeacon.net, port 7000 ___ UR RST IS ... ... ..9 QSB QSB - hw? BK
