Please note, the NRD-515 is a _receiver_, not a _transceivers_, thus most likely operated without a resonant (matched) antenna.
When operated with a matched antenna system, a bad noise figure is not a problem.
However, when operating with very bad, non-matched antennas, a good preamp noise figure is often required.
I also have their JST-100 which is a shortwave transceiver. Similar kind of RX in there and also has a very good NF. What many people overlook is that one does not necessarily use the same antenna for receive as for transmit. For example, to be able to null out an interferer. At lower frequencies that only works with small loop antennas and they don't offer a lot of signal. This kind is often passive:
Also, some folks use their transceivers in cars where the typical antenna length cannot exceed 10ft by much.
-- Regards, Joerg http://www.analogconsultants.com/
Some of the hams who work "top band" (1.8 - 2 Mhz) prefer to use split antenna systems. One guy I know swears by the combination of a vertical (for transmit) and a Beverage antenna (for receive). The latter is quite directional, and thus good at rejecting a lot of nearby noise sources that a vertical would pick up, but it's very lossy. A decent low-noise antenna helps bring the signal up to the point at which it's dominated by noise sources outside the receiver (atmospherics and man-made).
You do not need a matched antenna for RX. In fact, often you simply won't get one. BT, both mil and ham.
Absolutely not. I have a ham radio license but not actively using it right now. I also served in the signal corp in the military. There it is at times very customary to have a separate receive antenna, as I outlined in a reply to Jeff. Examples:
You are out in the field and can only build up a reasonable big TX antenna near your truck. Unfortunately that's also where the spark plugs of your generator rattle, where Joe runs his noisy electric razor to look good when he'll meet the women in town, and so on. This situation can be mitigated by having another small RD antenna up the hill. But it's a scary climb, so no big stuff.
Other situation: A multi-band multi-station ham radio contest. A plethora of antennas overhead, all energized to the brim. If you'd receive on one of those while Leroy next door keys his microphone ...
*PHUT* ... gone is the RX. Even if it survives it'll be nearly snuffed out during transmissions. The solution is the same as above, smaller RX antennas farther out.Yet another situation is cramped quarters. In ham radio as well as in military situations you often must remain mobile at a moment's notice. Either because it's a contest rule or because the Lt.Colonel said so. That means you can only use a vehicle whip antenna and that's not going to produce a lot of signal.
Actually, most good comms RX do not have much of a selective front end. They don't need it.
Would be nice but mostly that's not done.
Or just make sure you have tons of dynamic range. I got kind of used to that because I've designed so many ultrasound systems that it comes out of my ears. There, a huge dynamic range plus a super low noise floor will make the difference between a dud and a smashing sales success.
It's a whole different world. One university professor taught (!) us that a grounded gate configuration for amplifiers is stupid. Then I brought him a schematic of a top shelf comms RX and his jaw dropped. My very frst project in industry contained ... a grounded gate FET as the
1st RF stage. It beat the daylights out of the competition systems.-- Regards, Joerg http://www.analogconsultants.com/
On a sunny day (Tue, 20 May 2014 14:00:09 -0700) it happened snipped-for-privacy@coop.radagast.org (David Platt) wrote in :
I am simplistic in the sense that I test the receiver this way: Tune to no station without antenna, get noise. Connect antenna, get noise. If there is no difference the receiver is bad. If there is a lot of difference the receiver is good.
I build my first SSB receiver back in the sixties, the IF and BFO was a VERON design (Dutch amateur radio club), used a XF9A 9 MHz xtal filter, and chips! (CA3028 diff amp as IF) and a CA3020 as audio output. The RF preamp was my design, I tried many circuits, and ended up with a JFET cascode, the mixer was a dual gate (one of the first), and the VCO was a JFET too, some other mixers and multipliers. You could tune them Americans on a 1 meter wire one after the other in the afternoon.
RF gain adjust, did not even have AGC. I think Jourg is wrong here, on the lower RF bands, as in the antenna tests above, the atmospheric noise is so much stronger than the receiver noise it makes no sense to have sub microvolt sensitivity, only on the higher frequency bands.
But then most of those CIA commie haters are way below average IQ. I have a nice Ranger RCI-2970 transceiver (100W SSB) for the higher shortwave bands and CB, you can see it here at the bottom of the page:
Its pretty good, but not as sensitive as the thing I build in the sixties.
So I bought 2 Tecsun radios on ebay. one with SSB. And those meet my requirements, had a long recording of weather charts on 8038 kHz last night that I will use for testing. Also good signal on 14 MHz, and also with only internal ferrite rod a strong signal on long wave 136 kHz (a German weather station, but different frequency deviation). Noise free!
BTW I like to listen to news the way it is presented by different power, or look at there websites, US, Germany, China, Russia, China has a lot of very powerful shortwave transmissions in English, anyways it shows different aspects or views of what goes on in the world, could prevent one from being another puppet. With the Ranger RCI-2970 I worked with all the globe, including South America, just a on GPA antenna. It is hard to build something yourself that is better than those commercial receivers transceivers, really. that would be a lot of work.
While I know that best power match is not the same as best noise match for VHF/UHF preamps, I would not care about this in HF amplifiers in a noisy environment.
For multi-multi style operation, you really need to discipline the transmitter phase noise. A medium power (100 W) amateur band specific band pass filter between the transceiver and linear amplifier (1 kW+) will take care of that. If this filter is switched on also on receive, it will attenuate greatly the transmitted signals on different amateur band on the same site.
They definitively need octave (2:1 frequency range) front end filters.
At least in Europe, with 250-500 kW practically on every frequency in the 49 mb (6 MHz) and 41 m (7 MHz) bands, you are going to suffer a lot of intermodulation products on 1.8 MHz (antenna is at 3/4 resonance) and 18 MHz (3rd harmonics). Keeping this 6/7 MHz mess out of the mixer will help a lot.
While a simple SBL-1 style diode ring mixer (+7 dBm LO drive, -6 dB loss) might survive quite reasonably when connected directly to the antenna, you definitively need some +17 dBm LO diode ring mixer or some active mixer, if you intend to put some preamplifier in front of it.
For a transceiver, high receiver front end power is not an issue (high power is needed for transmission anyway), however for a receive only, especially if intended to be battery powered, the front end power consumption is a serious issue. Thus, building a strong front end with
1 h battery life is not an option.
At low HF and MF/LF with any decent antenna, you really need that attenuator.
For some reason, most high quality receivers seem to pick only a few stations, while those "super sensitive" receivers pick a lot of stations (=spurious responses :-).
How much over 100 dB are we talking about ? What bandwidth ? What tone separation ?
I am fully aware that various companies are bragging about their performance by selecting "suitable" parameter settings, but unfortunately the laws of physics might cause some problems :-)
Grounded base is nice to get _power_ amplification even after current amplification (fT) drops below 1.
You may not, but many potential buyers such as myself do.
The reality can be different. "Hey, Joe, can you bring your transceiver? Jim just blew the final transistors in his". Then you get to the site and it's "Oh dang!". Better to have separate antenna sites from the get-go.
Some do but those won't help in many cases because the din from most other signals concentrates largely inside one of those octaves. Or in some of my past cases mostly in one ham band.
+7dBm mixers would be considered sub-par by many. Even on 144MHz it is. I have a FT-225RD and the first thing the first owner did was rip out the front end and install a very high dynamic range one. That was one of my motivators to buy it from him.In an ultrasound Doppler design I have never used a mixer this wimpy. They'd have thrown the book at me. Those Dopplers are essentially just glorified shortwave receivers with some extra stuff such as range gates. The only thing that can replace a high LO drive is ... more LO drive.
In a contest that doesn't matter because you are transmitting 30-40% of the time. Anyhow, I'd never sacrifice dynamic range for battery power in an HF transceiver. Then I'd get a bigger battery :-)
With my RX's I normally don't. Except when I am hunting noise inside the front ends of medical ultrasound machines while they are pulsing. But that even brings an HP-3585 to its knees.
You need to buy a good one or take a mediocre RX and build a front end yourself. Unfortunately some of the best mixers for building one have vanished. I quickly put in an oder for a few dozen SL6440 the millisecond I heard Plessey was going to croak.
Easily 10-20.
Typically more than half of TX frequency. For example, if you have a
7.5MHz transducer as is often used in cardiology then the near field will be roughly 5MHz to 11MHz. In the far field you'll start to roll off the post-filter, sometimes all the way down to 8MHz. But in the far field dynamic range is no longer an issue. We call this a tracking filter because the BW is scooted down while echoes are coming in, on the fly.Pre-filtering: Zero, zip, nada. All wide open.
There is no tone separation in ultrasound. The most challenging situation is pulsed Doppler or Color Doppler in the near field. You get humongous echoes from hard structures such as bones of seriously calcified vessel walls. These echoes are of no interest to the cardiologist but must be properly dealt with by the machine. Pulse repetition frequencies are 2-15kHz, mostly, depending on depth, patient size et cetera. So you have several MHz worth of spectrum plastered with (sometimes weak) Doppler return signals that must be folded back into baseband by a range gate. If any of this isn't up to snuff in terms of intermodulation or dynamic range you can lose your shirt in this business.
No bragging here. It's all duked out "on the racetrack". In this case in side by side machine comparisons in major clinics by famous docs (we often call them luminaries). For example, they are going to try to see (or in the past hear) ever so slight jets in the blood stream which could point to a heart valve issue or other potentially dangerous conditions. If a machine is lacking in some aspects they quickly lose interest in it, the team is sent packing, and if this happens at more than one site word gets around and the product becomes a dud.
It is commonly used for receive purposes because such amps can handle lots of power while providing a good noise figure at the same time. The professor completely did not understand this. Afterwards I no longer attended the classes (wasn't mandatory) and just sat for the exam, answered in it what they wanted to hear, passed, and went on with life :-)
-- Regards, Joerg http://www.analogconsultants.com/
No it has not as few million licensed radio amateurs will conferm!
-- M0WYM Sales @ radiowymsey http://stores.ebay.co.uk/Sales-At-Radio-Wymsey/
Not a few million but rather about 723,000 licenses in the USA. The real number is probably much less because many hams are inactive. In addition, the FCC doesn't really update their database until 2 years after the 10 year renewal deadline, so there are many deceased hams still listed. If you run the "Report Generator", and only check "USA Totals", you'll get a graph showing the effect of dropping the Morse Code requirement. In the last 8 years, it shows a consistent increase of about 8,000 new hams per year.
-- Jeff Liebermann jeffl@cruzio.com 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558
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