+++++++++++++++++ Date: Tue, 17 Sep 2002 10:06:28 +0200 Organization: http://freemail.web.de/ From: Christoph Rheker To: "Elecraft" , "WayneBurdick" Subject: Re: [Elecraft] low-noise 40m RX antenna? Hallo Wayne, take a look at K9AY Loops, there was an article in QST some years ago. Also there was a good article abt los band RX antennas in QST, I think it was written by K6SE. The best resource is ON4UN's Low Band DX book. 73 de Chris, DL4YAO Wayne Burdick schrieb am 17.09.02 08:44:52: > I recently had to erect a temporary random-wire antenna, thanks to the > less-than-skilled tree trimming that brought down my real one. The > stand-in is shorter and much closer to noise sources on my property. The > result is 1 to 2 more S-units of noise on 40 meters and some other bands. > > Since the K2 has a separate RX antenna jack (on the K160RX module), I'm > thinking about putting up a low-noise receiving antenna for the low > bands. Unfortunately, to get enough wire for a Beverage antenna, I'd > have to sneak something along the fences in the power company easement > behind my house. This leads me to wonder about other possible low-noise > antenna solutions that might take less room (tuned loops, etc.). 40 > meters is the band I'm most interested in. > > Any suggestions? I think this topic might be of general interest for > weak-signal DX and QRP work on 40, but you seldom hear any discussion > about low-noise antennas outside of their application to 160 meters. > > tnx > Wayne > N6KR +++++++++++++++++ Date: Tue, 17 Sep 2002 07:22:48 -0700 To: From: "Gene A. Williamson" Subject: [Elecraft] low-noise 40m RX antenna I have found a loop to be a good low-noise receiving antenna. Up about 15ft and made of #20 magnet wire, it was nearly invisible. I fed it with ladder line to use as an all-band transmitting antenna. I would think coax feed with perhaps a QRP transmatch (if needed) would work well for RX only. Before you start climbing trees, try this: leaving the AGC on FAST, lower your RF gain until the S-meter needle rests at the same level as your noise. (Easiest done by going past the noise, then cranking the knob back up a hair.) Your noise will be gone, and you'll still have AGC action on signals louder than that level. Signals below that level will effectively sound as though you've set AGC OFF. My S-meter has rested at S5 for years now, and I'll bet I've made hundreds of QSOs where the signal didn't move the needle one whit. 73 Gene N7YW ++++++++++++++++++ Reply-To: From: "Ron D'Eau Claire" To: "'Elecraft'" Subject: RE: [Elecraft] low-noise 40m RX antenna? Date: Tue, 17 Sep 2002 07:29:02 -0700 I have a 2nd rx jack awaiting an alternative receive antenna too. One antenna that I am considering is a small receiving loop that I can rotate to null both QRN and QRM. The other is a subsurface (underground) antenna. John Heys G3BDQ in his little book "Practical Wire Antennas" discusses work with these. Of course they are no great performers on transmit producing signals about 16 dB below a "normal" dipole, but for receive... here's what G6PG reported, "...reduced QRN to almost zero and allowed the reception of shortwave signals almost as efficiently as did his elevated wire." All the reports seem to agree that the QRN is greatly reduced using an antenna on or underground. The antenna Heys discusses is a dipole, but very short by "normal" standards because of the loading effect of the earth. A 5 MHz dipole used by W0YBF was only 46.6 feet long, compared to 93.6 feet for a resonant dipole above ground. DeMaw also reports very good reception with 'on-ground' antennas. In his book, "W1FB's Antenna Notebook" he describes using an on-ground antenna consisting of a length of 50 coaxial line terminated at the far end with a 51 ohm resistor. A less-expensive alternative that worked as well, DeMaw writes, is a length of speaker wire terminated in a 220 ohm resistor at the far end and a 4:1 balun at the rig end. DeMaw wrote, "Even a random length of wire placed on the ground is often quieter than the transmitting antenna." What all of these antennas have in common is the need for a preamp -something in the order of 20 dB -- just what the K2 has already built-in and ready to go to work on the lower bands. And, with these, if you did encroach on the power company's easement a bit, you wouldn't have to worry about a hot line falling on your antenna in the event of an accident or of them tearing anything down (Now, tearing your antenna UP might be a problem if they dig trenches...). Ron AC7AC K2 # 1289 +++++++++++++++++ From: "K7FD N7SG" To: elecraft at mailman.qth.net Subject: Re: [Elecraft] low-noise 40m RX antenna Date: Tue, 17 Sep 2002 08:04:12 -0700 I also run a loop...a 40m vertical loop fed at the bottom, about 12 feet off the ground. A quarter wave matching section of 75ohm RG-11 coax provides an excellent match (1.06:1 at 7.040MHz). For cross country communications, it is the best antenna I have used on 40m and low noise is a definite benefit. 73 John K7FD +++++++++++++++++ To: n6kr at elecraft.com Cc: elecraft at mailman.qth.net Date: Tue, 17 Sep 2002 15:10:09 +0000 Subject: [Elecraft] low-noise 40m RX antenna? From: k6iii at juno.com Wayne, I am building up the PENNANT antenna for 160m receive. I think I will find that this is a low noise antenna. It is quite small (for 160m) in size in that it is only 29ft long by 14ft high and is directional. I am about ready to go with it but have not been able to locate a type-43 toroid to wind a matching transformer. This toroid needs to be about 3/4-inch diameter. It also requires a 20-30 db preamp. Another feature is that it can be low to the ground without much affect on its receiving performance. You can find info on web sites by searching on PENNANT 73 de Jerry/k6iii San Jose, CA +++++++++++++++++++ To: n6kr at elecraft.com, elecraft at mailman.qth.net Date: Tue, 17 Sep 2002 08:43:35 -0700 Subject: Re: [Elecraft] low-noise 40m RX antenna? From: k6se at juno.com In answer to your 40-meter noise problem, a simple loop antenna might be all that is necessary to null out the noise, but that antenna is bidirectional and does not have a receive S/N ratio as good as a unidirectional antenna. If you want a unidirectional receive antenna (which will also null out the noise), try the July 2000 QST article on "Flags, Pennants and other Ground-Independent Receiving Antennas". These antennas work well on 160m thru 30m according to reports and are popular for DXpedition use on the low bands. Another thing that may be worthwhile would be to use a noise-canceller to null out the noise. I understand from topbanders that the MFJ unit is the best one on the market. 73, de Earl, K6SE ++++++++++++++++++ From: Gerald Pfaff Reply-To: Gerald Pfaff To: elecraft at mailman.qth.net Date: Tue, 17 Sep 2002 20:51:03 +0100 Subject: [Elecraft] low-noise 40m RX antenna Wayne Burdick, N6KR wrote: I think this topic might be of general interest for weak-signal DX and QRP = work on 40, but you seldom hear any discussion about low-noise antennas = outside of their application to 160 meters. There are several commercial made =93magnetic loops=94 for 40m. I am using = the MFJ model. The big advantage is the selectivity. It acts like a = =93big=94 preselector. The disadvantage is that you have to re-tune every = few kHz. Once you get the hang on it it works really fast. The good thing = about it, you do not need a separate TX-antenna. K2 # 1185 with KPA100 and = the magnetic loop work just fine. (HB9 to W1 =96 W4 QSO=92s on 40m. Well, = those W-stations where not exactly QRP in the WAE-Contest ;-) Most of the = credit for these contacts are due to their antennas. There is a simple receiving antenna; too. It is a loop made out of RG213 = coax cable. The shield is split on top of the antenna. At the bottom both = split shields are connected to the shield of the coax to the RX. The inner = wires connects to a capacitor at the bottom of the loop. One side is = directly connected to the inner wire of the coax to the RX. Since you do = not transmit just peak the signal and do not worry about SWR. As I recall = it is described in an ARRL Antenna Handbook. Construction is cheap. Just = some feet of coax, a variable C and a supporting structure.=20 This antenna gives equal characteristics like a magnetic loop. But you need = to peak it for the frequency of interest. If this antenna is out of reach = you need to tune it remotely. I used this antenna just on the desk in the = shack with good results. The signal strength may not be what you expect with a full size dipole. But = QRM is reduced on this antenna. And when used standing on one side you can = null out, or reduce, unwanted signals. Laying the antenna =93flat=94 should result in an omnidirectional pattern. = This could match the pattern of a Groundplane / Vertical antenna for = transmitting. I did not try this. 73 de Gerald, HB9iRF ++++++++++++++++ From: "Stuart Rohre" To: "Wayne Burdick" , "Charles Greene" Cc: "Elecraft" Subject: Re: [Elecraft] low-noise 40m RX antenna? Date: Tue, 17 Sep 2002 17:58:51 -0500 The greatest low noise and gain antenna is a horizontal loop, if you have the room for a wave length of wire. It has both low and high angles of take off. If you use a transmatch, you could put up less than a wavelength at 80m and just feed it with parallel line, and a transmatch. Verticals can be a noisy antenna if mounted too near the ground. Most urban noise is vertically polarized. Vertical dipoles are best, vs. quarter wave radial ground verticals. 72, Stuart K5KVH +++++++++++++++++ From: "Stuart Rohre" To: "Wayne Burdick" , "Elecraft" Subject: Re: [Elecraft] low-noise 40m RX antenna? Date: Wed, 18 Sep 2002 15:18:41 -0500 Also, a point was made in a recent QEX or QST that for receiving only, you do not have to have a tuned loop, to get the low noise advantage it offers. Anything around a one wave circumference would be fine. With a transmatch and parallel line, even TV twin lead, a shorter than one wave could be pressed into use. 72, Stuart K5KVH ++++++++++++++++