HFU HF Underground
Technical Topics => Equipment => Topic started by: RadRoadrunner on September 25, 2016, 2159 UTC
-
I have recently bought one of the LuLu transmitters from eBay that broadcasts with a 10W carrier, 40W peak. Would like to find a cheap, lightweight antenna that won't produce a high SWR reading when tuned. Any and all suggestions are appreciated.
-
A dipole cut to the operating freq and in the clear will be very hard to beat for efficiency, a vertical with ground plane may get you a lower angle of radiation, however.
-
Dipole cut to length set up flat-top or inverted V style. They work well and the heaviest part is the coax. As close to foolproof as you can get. Stay away from power lines and try to get it up at least a 1/4 wavelength or higher if you can. Good listening antennas, too.
-
Are there any kits or plans for this? Also, maybe a center loaded dipole?
Dipole cut to length set up flat-top or inverted V style. They work well and the heaviest part is the coax. As close to foolproof as you can get. Stay away from power lines and try to get it up at least a 1/4 wavelength or higher if you can. Good listening antennas, too.
-
Go to any general ham radio site or do a search on "dipole antennas". There's plenty of info and step by step instructions. Putting together a dipole is like chewing gum and walking, just about anyone can do it.
-
468/f, gives a Half wave dipole in feet and inches, so 67'10".
Cut it in the middle and insert coax.
You MUST USE AN SWR METER to check.
The impedance of the antenna will change depending on it's height above ground.
So install (temporarily) at final height, check VSWR, adjust etc, OK.
VERY Easy!
Inverted V is probaly the best compromise (local and DX).
Low (less than 40') dipole is great for local via NVIS.
Best for DX is a dipole higher than 1/2 wave so 70' +
Better still a vertical with lots of radials or a few raised (tuned ones).
That's it!
Inv V is simplest so try that first, just look for ANY 40m antenna diagrams and make the antenna longer.
It's VERY handy to have a (HAM) radio that can TX on any band as you can see where the antenna is resonant.
So if it's got a low SWR on a lower frequency you know the antenna is too long etc, simple stuff?!!
The antenna is just a piece or wire, it's NOT magic!!
Let us know what you decide to do!
Str.
-
http://hamantennas.blogspot.co.uk/2011/02/simple-antenna-for-40-meters.html
-
I once read an article by a guy into QRP on a very similar antenna he used for dxpeditions in the boonies with the exception being he fed with 300 ohm TV twinline and used a small tuner, Stretchy. It was tapped in roughly the same spots on the coil.
The only reason he used the TV twinline was he could fit it and the dipole legs easily inside the coil form for storage in his pack. He used it in a variety of ways, vertical dipole, sloper, upright "L" (one of my favorite dipole variants), inverted "L" along with the standard "V" and flat-top setups.
At that size you could build two, one without a feedline, and set them up as a beam to the area you wanted to broadcast at. I'm having a brain fart at the moment, what's the spacing between the elements on wire beams, a tenth of a wavelength?
-
0.1 is about right but depends! can go as much as 0.2.
Did much piddling round with a 11m 4 el quad in the 80's.
That was fun!
SNORT!
;)
-
I'm having trouble in finding an ideal SWR meter
-
I'm having trouble in finding an ideal SWR meter
that really depends on how much you are willing to spend on one.
-
Just dug up an old part, it's expensive, but would it do the job? It's for 40 meters, and has an impedance made for up to 200 watts.
http://www.mfjenterprises.com/Product.php?productid=mfj-1621
-
That's a TUNER, NOT an SWR meter, which BTW are dirt cheap.
Forget any kind of antenna tuner, put up a proper antenna, 68' of wire cut in the middle and fed with coax.
Is it really that difficult!
Not sure what you mean by 'impedance made for up to 200 watts'
?
impedance is measured in Ohms, Power in Watts.
Anyway don't bother with some expensive tuner, put up a proper antenna.
PLEASE!
If you don't the signal will barely get outside your garden rather than the 1000 miles it was intended for!
This is the simplest but yet MOST IMPORTANT part of any transmitter system.
Maybe I should supply antenna with every system??
-
The problem I am now having is in terms of land space,
Now to make the dipole I want to make is 67 feet, I don't have have that space easily available in relation to where I will place the transmitter.
I know I can do this instead, but I want to make it clear. For a inverted V antenna is the total wing length half a wave length meaning two quater wave lengths?
-
Yep. The legs of the dipole in "V" don't have to be that far apart, 45-50 feet between the tie off points will get you in the ballpark for good performance if you get it up 35 feet at the center. It's dead simple.
You're looking at a foot over ten yards for each leg, it's not that much distance, flat or V shaped.
-
Well I ran a LULU through the antenna, thankfully nothing popped circuit board wise.
Though the MOSFET of the LULU does get hot it does not get hot enough to ignite paper.
I made an inverted V but had to eyeball the distances between the legs, because each leg was over a walkway path.
The antenna I made was based off of this
http://www.ebay.com/itm/40-Meter-HF-Dipole-Ultralight-Ultraportable-QRP-Stealth-Base-1-2-Wave-Antenna-/162200638040
Sadly I'm not producing range, and am considering adding an antenna tuner to see if I can boost a range.
I'm sure the physics gods, know a way to make a perfectly resonant antenna that produces range in a smaller package for HF.
-
How high are you getting the center up and are you using good coax, Arch? Not foil insulated TV type coax, but copper jacketed, low loss RF quality coax with quality connectors.
-
I want to say approx under 25 feet.
The cable I'm using is 50 ohm coax, I don't want to cut into it in order to find out it's copper sheilded
https://www.amazon.com/50ft-Rg8x-Pl259-Antenna-Cable/dp/B00D66RDYQ
Right now I'm feeling I'm going to have to erect a mast on my roof to get any range.
It's as if I'm still using my wire antenna.
Also my SWR is off the charts, then again I can't find the manual on the SWR-1 meter, so I brought another one off of ebay that includes it.
-
Can you either describe or post some photos of how you connected your coax to the center of the dipole? One of the most basic ways to build a dipole can be found here: http://www.cdagro.com/general/dpo.html (http://www.cdagro.com/general/dpo.html)
Basically, you need 3 insulators (one for each of the two ends and one for the center). The insulators don't have to be store bought, you can make some out of PVC pipe. You connect the braid of the coax to one leg, and the center conductor to the other leg. A clear diagram of this can be found on the ARRL website:
http://www.arrl.org/images/view/Licensing__Education_/Getting_on_the_Air/ARRL0113.jpg (http://www.arrl.org/images/view/Licensing__Education_/Getting_on_the_Air/ARRL0113.jpg)
What are you suspending the antenna from, and is the antenna relatively in the clear?
The original Free Radio Network website had some great technical information. Fortunately, some of it is still accessible via the "Wayback Machine". You should probably read the entire "Pirate Radio Survival Guide", one archived link is here:
https://web.archive.org/web/20040603210416/http://www.frn.net/special/prsg/ (https://web.archive.org/web/20040603210416/http://www.frn.net/special/prsg/)
-
Rg-8x is the good stuff, no need to cut it to look at the braid, you can see it one of the smaller pics at the top.
I've always had dipoles give high SWR when using a stock fifty feet length of coax as the feed. I've found a length of 65-75 feet to be optimum for transmitting. Probably an impedance thing. With 50 feet I've got to go with a tuner, in the mid-60's to mid-70's length it matches up fine no tuner needed. The tx barely gets warm to the touch. Luckily it's easy to find pre-rolled coax in 15 or 20 ft. lengths for sale. Get a female to female SO-239 to join the two sections together and you're in business.
The mast should help do the trick. If you can get 30 plus feet of height at the center point your results will start improve. Getting good results out of low powered transmitters is as much an art as it is a science. People tend to expect too much out of them. The guy's who use them with good results generally have a lot of frustration to begin with, then start to learn how to maximize performance. The most important thing I learned was time of day to use the things. You've got to hit the air when propagation is at an optimum for your low powered signal to be heard the best where you want it to be heard at. These low power tx'es are generally optimal for one hop tx'ing, out to about 600-800 miles. They can really pop if conditions are good and you time it just right.
Of course, you can be like me and transmit from a big hole in the ground and not give a damn, but most people like to be heard.
-
Here's an update, I have managed to get at least a mile in some parts of my neighborhood.
I didn't quiet know how to use the SWR-1 meter because I didn't have a manual so I ordered the ASTATIC PDC2 and pretty much learned off of that. Another problem I had with the meter was I was using a 50 foot coil of coax to connect the TX to the meter, I knew this would throw off the reading so I ordered a small 3 foot section of coax.
What my readings turned out to be was when I had no form of tuning I had MAX power of 20 watts but an SWR of 2:1
When I added an variable cap, the SWR dropped to 1.5:1 but in doing this I lost power. I also was very pleased to get a little bit of an RF burn when handling the varicap.
I probably won't post picture of my antenna set up as I may just scrap it and try again with another set up.
I definitely think now I need a mast, because one half side of the antenna is against a wall, which I'd imagine at this low of a frequency there is some absorption done by the house
I live it a densely populated suburb of NYC, and my goal is to at least hit deep into southern New Jersey.
-
Don't worry about how far away you can hear it close in, HF propagates by skywave, it goes up bounces off the atmosphere and comes back down in a wide coverage pattern a few hundred miles out.
Keep an on the logging's board for people hearing things on the frequency you're using for the time frame you're tx'ing in. A lot of these guys use SDR's on the funny band nearly 24/7. If you're getting out they've probably noted it. It will give you an idea of where your signal is going.
-
Keep an on the logging's board for people hearing things on the frequency you're using for the time frame you're tx'ing in. A lot of these guys use SDR's on the funny band nearly 24/7. If you're getting out they've probably noted it. It will give you an idea of where your signal is going.
It's funny you should mention that because I think I have been logged.
-
There you go, it works! Doesn't it feel good?
-
There you go, it works! Doesn't it feel good?
Yeah haha, I just wanna make a better set up.
I'm impressed that if the time is correct I was just above 2 watts of power when I was logged.
Now I can't wait a proper antenna at 20 watts would accomplish.
Thank you stretchyman who made this possible with his LULU TX package
-
Keep an on the logging's board for people hearing things on the frequency you're using for the time frame you're tx'ing in. A lot of these guys use SDR's on the funny band nearly 24/7. If you're getting out they've probably noted it. It will give you an idea of where your signal is going.
It's funny you should mention that because I think I have been logged.
You could let the lucky listener(s) know with a QSL ;D
-
Cool, OK Piggy is most correct as you wont hear anything locally.
NVIS and SKYWAVE are the ONLY propagation methods in use here.
So with NVIS mount a horizontal dipole at less than 30', you wont hear anything for a few 100 miles then from 200 to 1000 miles it should be reasonably strong.
and SKYWAVE use a vertical and dont expect to hear anything under 1000 miles.
However conditions vary wildly so these are ballpark figures
Oh, on checking SWR, you dont need a manual and 2:1 is fine, leave well alone.
-
Hey,
I thought a while whether I should write this or not. Primary because I have no intentions of annoying someone. But, well, I think I should say the following...
I've always had dipoles give high SWR when using a stock fifty feet length of coax as the feed. I've found a length (...)
Thank you for writing this. There is no use in voicing statements like "it always works" or "this is the perfect antenna".
A phrase like "this always works" will only lead to one thing: the freshman feeling stupid because he can't just get a simple works-always solution working. He'll feel frustrated. Just remember when we started? How many "why does that not work, damn?!"- moments did we live through? Especially with such complex things like antennas. And, well, ask 5 HAMS, gets 7 answers (which is fine. Everyone can only reflect from personal experiences, which differ greatly).
Let me explain why:
- An antenna exists (as experienced user know) of a an inductive (L, +j), capatitive (C, -j)and two ohmic parts (Wire loss Rl and radiation resistance Rr). Those parameters are prone to be affected by a lot of factors. Do we know about those factors at the place where the feshman wants to build his antenna? Hardly.
- Of course we can say such like "start with a wire that's a bit too long and cut it inch by inch until you hit the best SWR". But likely the poor guy will never obtain a good SWR. Why?
- The reactive L/C parts need to cancel each other out. That's the case when the wire is cut to the perfect length. But a perfect length does not grantee a perfect SWR because the feedpoint impedance may still be way off our target (lets say that's Z=50R). So the poor lad can start with 200 feet of wire and end up cutting it down to 1 inch and will still wonder "why I didn't get a good SWR? People say 'I just need to cut it down until I get a perfect SWR'".
- Maybe the freshman obtains a perfect SWR with testing the antenna 3m above the ground (so he can work on it with a ladder). As soon as he puts it up to 10m the SWR goes postal again. He doesn't know why and will be frustrated. Why? Because (we assume L/C cancel each other out) the feedpoint impedance swings (on a graph) up and down with rising height of the antenna. At some point it's 50R for sure. But likely it well be something between 20 and 80R.
- Maybe he uses 7m of Coax when experimenting and ends up with a good SWR. As soon as he uses 12m of Coax the SWR runs out of scale. Why? Because, yes, like Pigmeat mentioned, Coax can work as a impedance transformation line (look up "stub lines" or "stub transformator" on HAM literature).
But likely, the poor freshman faces some impedance like 36R+j155 and he has noooooo idea how to get that to R=50Z because no one gave him a kickstart with antennas but just "
this always works".
So, since I'm a HAM I'll add my own opinion to that now (I still have to stick to stereotypes). :D
So, what's the solution? No "just do this, it'll always work" for sure.
Of course, one of the best ways to get the freshman started is to provide him with some links to basic Antenna theory and advice him to puracse an antenna analyser (yeah, owies, several hundred bucks). But that way he can experiment and verify the theory from the book by results from the antenna analyser. And he can learn what stuff affects the antenna and will get it on track.
But, yeah. You're right to laugh at me for saying that. That's not realistic. :)
So my advice is: yes, use a (not complex, self build) Antenna Tuner. The setup is as follows:
[TX]-- Z=50R COAX-LINE -->[C-L-C Tuner] -> [BalUn] -> Dipole
Or, if it's prefered to have the tuner inhouse, he could use something like a G5RV-Antenna.
You may ask "why the tuner?". Well, like I wrote above, there's no way to know about the parameters of the antenna and a tuner is the simplest way to compensate any L/C-part and also transform the impedance to 50R.
Kind greetings (and really, I didn't mean to step on anyones tail if I did by accident),
Zazzle[/list]
-
Keep an on the logging's board for people hearing things on the frequency you're using for the time frame you're tx'ing in. A lot of these guys use SDR's on the funny band nearly 24/7. If you're getting out they've probably noted it. It will give you an idea of where your signal is going.
It's funny you should mention that because I think I have been logged.
You could let the lucky listener(s) know with a QSL ;D
Radio A.V. advises his e-mail is radioA.V@outlook.com
He has been logged twice before we logged him yesterday (10/31)
https://www.hfunderground.com/board/index.php/topic,30795.0.html
In fact this thread needs to be updated to reflect Radio A.V. instead of UNID:
https://www.hfunderground.com/board/index.php/topic,30750.0.html
-
Coax wise you need to use an ODD number of half wave lengths (X the velocity factor of your cable, usually 0.66)
Read here;
http://www.radiomods.co.nz/coaxlength.html
Bit CB'ified but just change the numbers.
A simple SWR meter can get confused and make it look like you have a poor SWR when you actually have a good one!
OH, Mr.ArchVile you need to use VHF if you want to TX 'Locally'.
S.W. will TX as described in my earlier post, min distance (with NVIS) is about 100miles, you'll here nothing between the TX and this minimum distance.