There are other accessories available at very very low prices. Splitters, barrels, splitters, etc.
Somehow the stuff reminds me of aircom-plus (€3.35 for 1m)... many times cheaper though...
Saturday, March 19, 2011
75Ohms Coax update
Low loss & cheap, that was just one part of the story about RG-6 sat-coax. The other side would be the (cheap) accessories one can get for the 75Ohms F-system. My secondary grabber system (Target HF3 & Yaesu FRT-7700) is hooked up to 75Ohms for a couple of days. Additionally I am using one of those ("Sat-Flex"-adapter):
For the readers who have never seen those before, such devices are use to lead a TV-sat signal from the LNC to the indoors sat-receiver. Those adapters are squeezed between a window and a window frame.
There are other accessories available at very very low prices. Splitters, barrels, splitters, etc.
And even better news. I not buying the cheapest available, one may be lucky and obtain this:
Well, it was slightly more expensive (€8.- for 10m @ HEMA) than the previous example, and it did not come with F-connectors. However, see the air-cells? Next to the Al-foil shield, there is a Cu-braid, the soul is made from Cu too. This cable can be soldered just fine.
Somehow the stuff reminds me of aircom-plus (€3.35 for 1m)... many times cheaper though...
There are other accessories available at very very low prices. Splitters, barrels, splitters, etc.
Somehow the stuff reminds me of aircom-plus (€3.35 for 1m)... many times cheaper though...
Monday, March 14, 2011
75Ohms, why not?!
I have been experimenting a bit with more or less cheap (inexpensive that is) materials lately. Came across something called RG-6, or, in other words, very inexpensive 75Ohms coax-cable for TV-satellite reception.
TV-sat-RX means, that losses at frequencies between 1GHz and 2GHz are acceptable. The cable itself proofs on that point, foam dielectric....
We amateurs are using stuff called RG-58, RG-213 etc. for ages.... This relatively expensive stuff is rated 50 (or 52) Ohms, as most of our transceiver's aerial connectors. Most of the modern times transceivers however still employ so called UHF-connectors... something sooo outdated, you would not believe it! And further, look at the inside of 'm transceivers, there is not 50Ohms match found anywhere.... So, what is that hype about 50Ohms anyway?! I don't know!
And hence, I don't care!
However, nowadays UHF-connector have a thread for the the shield. The soul of the sat-coax is copper and hence can be soldered easily. There is hence an easy way to adapt 50Ohms PL-connectors to 75Ohms F-connectors.
My local hardware store provides me with satellite-grade 75Ohms cable for a price of about €5.- per 10m including two of the so called F-connectors... a price, RG-58 cannot match, not to speak about the (two) UHF-connectors aka PL259.
Let's further search for justification of using more expensive 50Ohms-stuff rather than 75Ohms-mass-ware. A dipole is mentioned to have an impedance of 60Ohms.... OK 50Ohms is marginally closer to that than 75Ohms would be.... but... does that matter? I don't think so!
Let's turn to the connectors for a change. I mentioned UHF-connector (aka PL-connectors) before. To the time these connectors have been invented, names like UHF were justified, I believe. However, in modern times, this merely reflects a relict from the glorious past... UHF connectors are considered good for shortwave and lower frequencies. Connectors considered suitable for UHF as of today would be BNC and N-connectors (both 50Ohms nevertheless).
Still 50Ohms, but why? I believe, we stick to 50Ohms (or 52Ohms) for reasons of tradition. Industry seems to like that, since there still is a reason to produce (and sell) 50Ohms coax like RG-58 and RG-213.
As radio-amateur, or HAM for good measures, you always want to watch your options.
I watched mine, and I came to the conclusion that, where ever I would need coax-cable the cheapest commercial (i.e. mass market) solution available was the best for the purpose. Hence I will use RG-6 or whatever (low loss) will be available for cheap at my local hardware store. Who needs specialized retailers, if a tiny change in the setup will do?
Up to now I was writing about low power and low-noise reception. Let's face the other option for a change... QRO! When facing high power, I would anyway not consider using coax-cables. Those just get hot on losses. For QRO, I personally would/will/do go for open wire feed-lines. If that appears too difficult to do, I still would propose using high ohmic window-line for the purpose.
Conclusion: use cheap satellite-TV-coax and forget about expensive 50Ohms stuff!
73!
TV-sat-RX means, that losses at frequencies between 1GHz and 2GHz are acceptable. The cable itself proofs on that point, foam dielectric....
We amateurs are using stuff called RG-58, RG-213 etc. for ages.... This relatively expensive stuff is rated 50 (or 52) Ohms, as most of our transceiver's aerial connectors. Most of the modern times transceivers however still employ so called UHF-connectors... something sooo outdated, you would not believe it! And further, look at the inside of 'm transceivers, there is not 50Ohms match found anywhere.... So, what is that hype about 50Ohms anyway?! I don't know!
And hence, I don't care!
However, nowadays UHF-connector have a thread for the the shield. The soul of the sat-coax is copper and hence can be soldered easily. There is hence an easy way to adapt 50Ohms PL-connectors to 75Ohms F-connectors.
 |
| F-connector 75Ohms coax and a 50Ohms UHF-connector |
My local hardware store provides me with satellite-grade 75Ohms cable for a price of about €5.- per 10m including two of the so called F-connectors... a price, RG-58 cannot match, not to speak about the (two) UHF-connectors aka PL259.
Let's further search for justification of using more expensive 50Ohms-stuff rather than 75Ohms-mass-ware. A dipole is mentioned to have an impedance of 60Ohms.... OK 50Ohms is marginally closer to that than 75Ohms would be.... but... does that matter? I don't think so!
Let's turn to the connectors for a change. I mentioned UHF-connector (aka PL-connectors) before. To the time these connectors have been invented, names like UHF were justified, I believe. However, in modern times, this merely reflects a relict from the glorious past... UHF connectors are considered good for shortwave and lower frequencies. Connectors considered suitable for UHF as of today would be BNC and N-connectors (both 50Ohms nevertheless).
Still 50Ohms, but why? I believe, we stick to 50Ohms (or 52Ohms) for reasons of tradition. Industry seems to like that, since there still is a reason to produce (and sell) 50Ohms coax like RG-58 and RG-213.
As radio-amateur, or HAM for good measures, you always want to watch your options.
I watched mine, and I came to the conclusion that, where ever I would need coax-cable the cheapest commercial (i.e. mass market) solution available was the best for the purpose. Hence I will use RG-6 or whatever (low loss) will be available for cheap at my local hardware store. Who needs specialized retailers, if a tiny change in the setup will do?
Up to now I was writing about low power and low-noise reception. Let's face the other option for a change... QRO! When facing high power, I would anyway not consider using coax-cables. Those just get hot on losses. For QRO, I personally would/will/do go for open wire feed-lines. If that appears too difficult to do, I still would propose using high ohmic window-line for the purpose.
Conclusion: use cheap satellite-TV-coax and forget about expensive 50Ohms stuff!
73!
Friday, March 4, 2011
6m QRSS
This is an extremely easy one. Most QRSS operators once built a 30m MEPT. There is an easy way to recycle those transmitters for 6m. With a converter circuit running a 40MHz oscillator, a 10.140MHz MEPT would be right away transmitting on 50.140MHz. Not too bad, I figure. A NE612 would probably be the easiest and cheapest way to get there this way.
Alternatively, a subharmonic mixer (two anti-parallel diodes) on a 20MHz oscillator would be even easier to build.
There are canned oscillators for 40MHz and 20MHz available, not to mention those cheap crystals.
Alternatively, a subharmonic mixer (two anti-parallel diodes) on a 20MHz oscillator would be even easier to build.
There are canned oscillators for 40MHz and 20MHz available, not to mention those cheap crystals.
Wednesday, March 2, 2011
2m QRSS
Something I was thinking of occasionally, QRSS on VHF, UHF, SHF and EHF bands.
Lets first start with a band that is accessible to all of us, the 2m band.
A short chat with Dave (G7UVW) brought up two frequencies of interest.
Both QRGs can be reached with CB-TX-crystals on their fundamental frequency and doubling the signals a couple of times. 9.x MHz will have to be multiplied by a factor of 16. With a little pull and the right oscillator circuit, this all should be fairely easy.
144.070 MHz
Channel 5T (27.015MHz) will be, very obviously, be 9.005MHz on the fundamental frequency. The resulting frequency on the 2m would be 144.080MHz. A little harmless down pull to 9.004375MHz on the fundamental will get us to 144.070MHz.
144.4905 MHz (WSPR-QRG)
We got two options here, channels 11T (27.085MHz) and 12T (27.105MHz).
11T would require a Pierce oscillator which oscillates above the series frequency of the crystal. The fundamental of 11T is 9.028333MHz, while the required frequency for the target-frequency would be 9.03065625MHz. The difference of 2.323kHz is not problem to a Pierce oscillator at all.
12T will provide a 9.035MHz signal. The pull of 4.344kHz to 9.03065625MHz is doable, could however result in a less stable oscillator.
Put the design into practice seem not to be a great deal, however, frequency stability is a great deal here. So, I figure, good temperature stabilization is in order. The rest is in fact pretty straight forward. I do have to admit that the first ever ham-radio transmitter I built (age 15, just licensed) was a 144MHz A1A TX, based on a CB crystal and BF199 transistors.
Lets first start with a band that is accessible to all of us, the 2m band.
A short chat with Dave (G7UVW) brought up two frequencies of interest.
- 144.070 MHz
- 144.4905 MHz (WSPR)
Both QRGs can be reached with CB-TX-crystals on their fundamental frequency and doubling the signals a couple of times. 9.x MHz will have to be multiplied by a factor of 16. With a little pull and the right oscillator circuit, this all should be fairely easy.
144.070 MHz
Channel 5T (27.015MHz) will be, very obviously, be 9.005MHz on the fundamental frequency. The resulting frequency on the 2m would be 144.080MHz. A little harmless down pull to 9.004375MHz on the fundamental will get us to 144.070MHz.
144.4905 MHz (WSPR-QRG)
We got two options here, channels 11T (27.085MHz) and 12T (27.105MHz).
11T would require a Pierce oscillator which oscillates above the series frequency of the crystal. The fundamental of 11T is 9.028333MHz, while the required frequency for the target-frequency would be 9.03065625MHz. The difference of 2.323kHz is not problem to a Pierce oscillator at all.
12T will provide a 9.035MHz signal. The pull of 4.344kHz to 9.03065625MHz is doable, could however result in a less stable oscillator.
Put the design into practice seem not to be a great deal, however, frequency stability is a great deal here. So, I figure, good temperature stabilization is in order. The rest is in fact pretty straight forward. I do have to admit that the first ever ham-radio transmitter I built (age 15, just licensed) was a 144MHz A1A TX, based on a CB crystal and BF199 transistors.
Monday, February 28, 2011
The T2SFM Antenna
The T2FD (aka TTFD, Terminated Tilted Folded Dipole) may be know to many of us. No, I am not going to discuss the T2FD here. It is just mentioned since it forms the basis for the following design considerations.
The only thing we should keep in mind, the span of a T2FD is 30% of the lowest wavelength, not 50% as a regular folded dipole would be. The distance of the fold is 1% of the wavelength.
In a country as densely populated as Holland, one needs to think of room saving antenna solutions regularly. The goal of the game is hence to reduce the span of antenna setups.
The aerial I am thinking of needs to the at least the popular QRSS bands 80m - 30m, possibly 160m and 20m.
First design step, the a quarter-wave radiator for 30m and tilt it. The length of such radiator would be 7.6m.
Second step, a trap terminating the 30m radiator. This trap also acts as loading inductor for the lower frequencies. As a fist assumption, I was randomly picking 10µH, the resulting capacitance is 25pF.
Third, the folding distance is chosen to be 50cm, reflecting an operation wavelength of 50m, which is somewhere between 40m and 80m obviously.
The last and remaining bit would be the second "T" of the name, i.e. the termination. I figured 600Ohms could be a good value to start from. When putting this into reality, one needs to remember that the termination resistor should not be induction and should be rated at least 30% of the power applied to the aerial.
There the MMANA script a hacked in:
80m - 20m T2SFM
*
10.12
***Wires***
4
0.0, 0.0, 0.0, -7.0, 0.0, 3.0, 8.000e-04, -1
-7.0, 0.0, 3.0, -7.0, 0.0, 3.5, 8.000e-04, -1
-7.0, 0.0, 3.5, 0.0, 0.0, 0.5, 8.000e-04, -1
0.0, 0.0, 0.0, 0.0, 0.0, 0.5, 8.000e-04, -1
***Source***
1, 1
w1b, 0.0, 1.0
***Load***
2, 1
w2c, 0, 2.17, 114.0, 0.0
w4c, 1, 600.0, 0.0
***Segmentation***
800, 80, 2.0, 1
***G/H/M/R/AzEl/X***
2, 0.0, 1, 600.0, 120, 60, 0.0
###Comment###
Mod by Joachim, PA1GSJ 2/28/2011 9:55:09 PM
Created by Joachim, PA1GSJ 2/28/2011 9:08:01 PM
The first simulations look promising, still a lot room for optimizations. Would be time to also start putting stuff together and try out if this contraption is any good.
REVISION
Had another thought about the 30m trap. The easiest way to make a trap would be a coax trap. With a diameter of 4cm, a length of 4cm a coax trap would have a self-inductance of 2.17µH and a capacitance of 114pF. Purely mechanical, 117cm of RG58 are required in order to form 8 turns on a 4cm diameter.
The only thing we should keep in mind, the span of a T2FD is 30% of the lowest wavelength, not 50% as a regular folded dipole would be. The distance of the fold is 1% of the wavelength.
In a country as densely populated as Holland, one needs to think of room saving antenna solutions regularly. The goal of the game is hence to reduce the span of antenna setups.
The aerial I am thinking of needs to the at least the popular QRSS bands 80m - 30m, possibly 160m and 20m.
First design step, the a quarter-wave radiator for 30m and tilt it. The length of such radiator would be 7.6m.
Second step, a trap terminating the 30m radiator. This trap also acts as loading inductor for the lower frequencies. As a fist assumption, I was randomly picking 10µH, the resulting capacitance is 25pF.
Third, the folding distance is chosen to be 50cm, reflecting an operation wavelength of 50m, which is somewhere between 40m and 80m obviously.
The last and remaining bit would be the second "T" of the name, i.e. the termination. I figured 600Ohms could be a good value to start from. When putting this into reality, one needs to remember that the termination resistor should not be induction and should be rated at least 30% of the power applied to the aerial.
There the MMANA script a hacked in:
*
10.12
***Wires***
4
0.0, 0.0, 0.0, -7.0, 0.0, 3.0, 8.000e-04, -1
-7.0, 0.0, 3.0, -7.0, 0.0, 3.5, 8.000e-04, -1
-7.0, 0.0, 3.5, 0.0, 0.0, 0.5, 8.000e-04, -1
0.0, 0.0, 0.0, 0.0, 0.0, 0.5, 8.000e-04, -1
***Source***
1, 1
w1b, 0.0, 1.0
***Load***
2, 1
w2c, 0, 2.17, 114.0, 0.0
w4c, 1, 600.0, 0.0
***Segmentation***
800, 80, 2.0, 1
***G/H/M/R/AzEl/X***
2, 0.0, 1, 600.0, 120, 60, 0.0
###Comment###
Mod by Joachim, PA1GSJ 2/28/2011 9:55:09 PM
Created by Joachim, PA1GSJ 2/28/2011 9:08:01 PM
The first simulations look promising, still a lot room for optimizations. Would be time to also start putting stuff together and try out if this contraption is any good.
REVISION
Had another thought about the 30m trap. The easiest way to make a trap would be a coax trap. With a diameter of 4cm, a length of 4cm a coax trap would have a self-inductance of 2.17µH and a capacitance of 114pF. Purely mechanical, 117cm of RG58 are required in order to form 8 turns on a 4cm diameter.
Thursday, February 24, 2011
Inexpensive Small Computer
Remember when I was writing about a "grabber viewer gadget" some weeks ago?
Well, the thing can be used for more, apparently, since it most likely was not build for grabber-viewing anyway ;-)
First, lets have a look what a minimal setup can look like...
The cost of the total setup is about €150.- for the ARCHOS 7 HT (8GB SSD, microSDHC, WiFi), €20,- for the wireless mini keyboard and €5.- for the USB gender-changer. The USB cable and the power supply are provided with the ARCHOS.
The only thing one has to do is to enable the USB HOST MODE in the ARCHOS' configuration menu. As soon as a pointing device is recognized, a mouse pointer is shown on the screen.
Amongst USB accessories I tested were a Micro Hub, a card-reader, thumb drives, a keyboard with built-in Hub, mice.
There seem to be ways to toggle to an alternative window manager. Android is perfect for touch-screens, operation, however when used with keyboard and mouse, the advantages of Android don't really help.
Well, the thing can be used for more, apparently, since it most likely was not build for grabber-viewing anyway ;-)
First, lets have a look what a minimal setup can look like...
 | |
| ARCHOS 7 HT |
The only thing one has to do is to enable the USB HOST MODE in the ARCHOS' configuration menu. As soon as a pointing device is recognized, a mouse pointer is shown on the screen.
Amongst USB accessories I tested were a Micro Hub, a card-reader, thumb drives, a keyboard with built-in Hub, mice.
There seem to be ways to toggle to an alternative window manager. Android is perfect for touch-screens, operation, however when used with keyboard and mouse, the advantages of Android don't really help.
Monday, February 21, 2011
Propagation JO29 --> JO22
Most interesting! Two Norwegian stations, LA5GOA and LA9BEA (neighbors), running MEPTs from JO29.
The locations of the two stations, as googled, are:
LA5GOA in JO29OI
LA9BEA in JO29OG
Steen Erik wrote on the KnightsQRSS list "LA5GOA and LA9BEA live about 15km apart on the island Karmøy on the west coast of Norway". Seen from my QTH, both stations share the same heading and hence, LA9BEA is by 15km closer to my location than LA5GOA.
And here comes the interesting part, not surprising however. Both transmitters create comparable field strengths in my humble grabber (located in JO22DA).
The following spectra were received by my 30m grabber setup: 30m-DCTL & subharmonic direct-conversion receiver. Note, the receiver does not employ AGC circuitry.
Please observe the different field strengths caused by the two friends and enjoy our ionosphere in action!
The locations of the two stations, as googled, are:
LA5GOA in JO29OI
LA9BEA in JO29OG
Steen Erik wrote on the KnightsQRSS list "LA5GOA and LA9BEA live about 15km apart on the island Karmøy on the west coast of Norway". Seen from my QTH, both stations share the same heading and hence, LA9BEA is by 15km closer to my location than LA5GOA.
And here comes the interesting part, not surprising however. Both transmitters create comparable field strengths in my humble grabber (located in JO22DA).
The following spectra were received by my 30m grabber setup: 30m-DCTL & subharmonic direct-conversion receiver. Note, the receiver does not employ AGC circuitry.
Please observe the different field strengths caused by the two friends and enjoy our ionosphere in action!
Thursday, February 17, 2011
Just an Idea
From VHF and UHF operations we know to use preamps at the antenna feedpoint as to improve SNR. SHF and EHF amateur radio operation is usually done by having the transverter at the antenna's feedpoint, since losses in the transmission line would eat up the signals completely...
So, what's the idea? Well, in QRSS, we also hunt for the best possible SNR. So, why not moving half or more of a QRSS station's circuitry to the antenna?
Half? Yes, I would prefer/advice to have frequency determining blocks, i.e. oscillators, in a controlled environment, e.g. the shack.
DC-Feed
The basic trick on all of the following would be feeding DC from the shack to the device at the antenna, whatever the device will be. Feeding is done in the well known way for active aerial or mast-head preamps.
TX
Easy things first, lets have a look at transmitters. The only bit that needs to be at the antenna feedpoint would be the final amplifier. The only thing to watch out for would be the Ohmic loss of the transmission line, since the final stage will require some current. If one cares about the voltage which the final is driven from, one should take this transmission line DC resistance into consideration.
RX
Reception is somewhat more challenging. Why do it in the first place? A preamp could do that. Yes, it could. However, some designs (e.g. w/ the NE612) do not really need pre-amplification...
Here's a look at the different cases:
Direct Conversion
In this case, the LO would be in the shack, and the whole rest of the receiver remote at the antenna location. It should not pose any problem to combine/split LO-RF, RX-AF and DC. However, the supply DC can carry mains hum residuals. A decent high-pass will be necessary before the AF can be fed into any sound-card.
Supersonic-Heterodyne (Superhet)
In such a scenario, front-end, mixer and IF filter (xtal-ladder) would be distal. LO, BFO, product detector and AF-amp would proximal. It is conceivable/advisable to add a second xtal-ladder to split IF and BFO frequencies. The downside would be the more complex frequency splitting at the distal part. A crystal filter would come handy here, and therefore, a "cheap crystal combination" would be recommendable for such a design, however tempting a DDS design would be.
RX&TX
Now to the tricky stuff... RX/TX-toggle could possibly done by polarity reversal, e.g. by diodes and relays.
I would like to leave it to your imagination what would be possible...
Conclusions
This may not be necessary to think about at all...
However, in case of reception, having all the low-noise stuff at the distal location, one may consider using cheap twin lead cable to connect the remote head with the shack.
For transmission, the obvious advantage would be that the final could be matched to TX-aerial, whatever it may be in balance and/or impedance.
So, what's the idea? Well, in QRSS, we also hunt for the best possible SNR. So, why not moving half or more of a QRSS station's circuitry to the antenna?
Half? Yes, I would prefer/advice to have frequency determining blocks, i.e. oscillators, in a controlled environment, e.g. the shack.
DC-Feed
The basic trick on all of the following would be feeding DC from the shack to the device at the antenna, whatever the device will be. Feeding is done in the well known way for active aerial or mast-head preamps.
TX
Easy things first, lets have a look at transmitters. The only bit that needs to be at the antenna feedpoint would be the final amplifier. The only thing to watch out for would be the Ohmic loss of the transmission line, since the final stage will require some current. If one cares about the voltage which the final is driven from, one should take this transmission line DC resistance into consideration.
RX
Reception is somewhat more challenging. Why do it in the first place? A preamp could do that. Yes, it could. However, some designs (e.g. w/ the NE612) do not really need pre-amplification...
Here's a look at the different cases:
Direct Conversion
In this case, the LO would be in the shack, and the whole rest of the receiver remote at the antenna location. It should not pose any problem to combine/split LO-RF, RX-AF and DC. However, the supply DC can carry mains hum residuals. A decent high-pass will be necessary before the AF can be fed into any sound-card.
Supersonic-Heterodyne (Superhet)
In such a scenario, front-end, mixer and IF filter (xtal-ladder) would be distal. LO, BFO, product detector and AF-amp would proximal. It is conceivable/advisable to add a second xtal-ladder to split IF and BFO frequencies. The downside would be the more complex frequency splitting at the distal part. A crystal filter would come handy here, and therefore, a "cheap crystal combination" would be recommendable for such a design, however tempting a DDS design would be.
RX&TX
Now to the tricky stuff... RX/TX-toggle could possibly done by polarity reversal, e.g. by diodes and relays.
I would like to leave it to your imagination what would be possible...
Conclusions
This may not be necessary to think about at all...
However, in case of reception, having all the low-noise stuff at the distal location, one may consider using cheap twin lead cable to connect the remote head with the shack.
For transmission, the obvious advantage would be that the final could be matched to TX-aerial, whatever it may be in balance and/or impedance.
Thursday, February 10, 2011
JUMA-RX1 kit arrived
Unboxing of a kit... I would usually not make any noises about. However, this time it seems appropriate to share this experience with you.
SRAT sent the kit in a solid cardboard box with more than sufficient padding. I received an email the day the kit was shipped, with tracking info! The shipment was tagged "1st class - PRIORITY" and "RECOMMANDÉ". I am impressed!
The 1st class treatment continues inside the box, carrying a bag containing a TEKO casing. The kit itself is contained inside the TEKO casing:
Note, the kit supplies you with everything you could possibly require... even rubber pads.
SRAT sent the kit in a solid cardboard box with more than sufficient padding. I received an email the day the kit was shipped, with tracking info! The shipment was tagged "1st class - PRIORITY" and "RECOMMANDÉ". I am impressed!
The 1st class treatment continues inside the box, carrying a bag containing a TEKO casing. The kit itself is contained inside the TEKO casing:
 | |
| JUMA-RX1 receiver kit |
Wednesday, February 9, 2011
JUMA-RX1 possible mod
To the time of writing this post., my JUMA-RX1 kit is still waiting with postal services. Will be pick the kit up tomorrow....
Time to look into some possible modifications. Why, you may ask, why the hack modify a well thought through design? Vy vy simple, the receiver, due to the simplicity of the design, is limited to bands 40m and lower, i.e. 100kHz to 7.5MHz.
So, what is that constraint all about? The design uses a 16F819 as controller. This PIC is limited to a 20MHz clock frequency. The clever bit of OH2NLT's design is to use the 16F819's clock for the AD9833 DDS too. This however limits the DDS output frequency. One possibility would be to have the DDS running at another clock frequency. Such a modification would however require reprogramming of the firmware, in order to adapt to the new DDS clock. Possible, ok, but somewhat intensive.
OH7SV designed a direct conversion receiver with an active switching mixer. Absolutely nothing wrong with this design. If we sacrifice the 136kHz band, we could gain the 30m and 20m bands by doubling the LO. The frequency coverage will move from 100kHz-7.5MHz to 200kHz-15MHz.
There is one downside to this game, the LO frequency steps are also doubled. The 10Hz minimal steps would result in 20Hz QRG change... also frequency readout will have to be doubled to determine the QRG.
Here would be my preferred solution: introduce a second front-end having a low-pass filter, a subharmonic mixer and a low noise op-amp AF preamp. A dual switch toggles the antenna input and the preamp output between the original and the additional circuits. Extracting signal to the subharmonic mixer may require an additional adjustable driver.
Such a mod will preserve 136kHz operation and better fine tuning in direct conversion mode and offer improved performance and wider coverage in subharmonic mode.
Should there be no need for 136kHz and/or 10Hz steps, the whole front-end could be modified easily by changing low-pass filter components and adding a frequency doubler between the DDS and the RX boards.
In case subharmonic advantages are on the wish-list, the MUX-mixer could be replaced by a RA3AAE-mixer (don't forget the low-pass!). In this case the differential preamp should be modified too.
I am sure that even more possibilities would be obvious to the skilled artisan.
Stay tune for practical experience, as soon as I picked up and built the kit.
Time to look into some possible modifications. Why, you may ask, why the hack modify a well thought through design? Vy vy simple, the receiver, due to the simplicity of the design, is limited to bands 40m and lower, i.e. 100kHz to 7.5MHz.
So, what is that constraint all about? The design uses a 16F819 as controller. This PIC is limited to a 20MHz clock frequency. The clever bit of OH2NLT's design is to use the 16F819's clock for the AD9833 DDS too. This however limits the DDS output frequency. One possibility would be to have the DDS running at another clock frequency. Such a modification would however require reprogramming of the firmware, in order to adapt to the new DDS clock. Possible, ok, but somewhat intensive.
OH7SV designed a direct conversion receiver with an active switching mixer. Absolutely nothing wrong with this design. If we sacrifice the 136kHz band, we could gain the 30m and 20m bands by doubling the LO. The frequency coverage will move from 100kHz-7.5MHz to 200kHz-15MHz.
There is one downside to this game, the LO frequency steps are also doubled. The 10Hz minimal steps would result in 20Hz QRG change... also frequency readout will have to be doubled to determine the QRG.
Here would be my preferred solution: introduce a second front-end having a low-pass filter, a subharmonic mixer and a low noise op-amp AF preamp. A dual switch toggles the antenna input and the preamp output between the original and the additional circuits. Extracting signal to the subharmonic mixer may require an additional adjustable driver.
Such a mod will preserve 136kHz operation and better fine tuning in direct conversion mode and offer improved performance and wider coverage in subharmonic mode.
Should there be no need for 136kHz and/or 10Hz steps, the whole front-end could be modified easily by changing low-pass filter components and adding a frequency doubler between the DDS and the RX boards.
In case subharmonic advantages are on the wish-list, the MUX-mixer could be replaced by a RA3AAE-mixer (don't forget the low-pass!). In this case the differential preamp should be modified too.
I am sure that even more possibilities would be obvious to the skilled artisan.
Stay tune for practical experience, as soon as I picked up and built the kit.
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