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Author Topic: Advice on choosing an antenna that won't make my transmitter go boom  (Read 5741 times)

Offline Mr.ArchVile

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Re: Advice on choosing an antenna that won't make my transmitter go boom
« Reply #15 on: October 27, 2016, 2320 UTC »
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. 




Offline Pigmeat

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Re: Advice on choosing an antenna that won't make my transmitter go boom
« Reply #16 on: October 28, 2016, 0300 UTC »
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.

Offline Mr.ArchVile

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Re: Advice on choosing an antenna that won't make my transmitter go boom
« Reply #17 on: October 28, 2016, 0321 UTC »
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.

Offline curious george

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Re: Advice on choosing an antenna that won't make my transmitter go boom
« Reply #18 on: October 28, 2016, 1731 UTC »
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

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

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/
« Last Edit: October 28, 2016, 1752 UTC by curious george »

Offline Pigmeat

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Re: Advice on choosing an antenna that won't make my transmitter go boom
« Reply #19 on: October 28, 2016, 2033 UTC »
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.

Offline Mr.ArchVile

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Re: Advice on choosing an antenna that won't make my transmitter go boom
« Reply #20 on: October 28, 2016, 2337 UTC »
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.

Offline Pigmeat

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Re: Advice on choosing an antenna that won't make my transmitter go boom
« Reply #21 on: October 29, 2016, 0031 UTC »
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.

Offline Mr.ArchVile

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Re: Advice on choosing an antenna that won't make my transmitter go boom
« Reply #22 on: October 29, 2016, 0045 UTC »

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.


Offline Pigmeat

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Re: Advice on choosing an antenna that won't make my transmitter go boom
« Reply #23 on: October 29, 2016, 0048 UTC »
There you go, it works! Doesn't it feel good?

Offline Mr.ArchVile

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Re: Advice on choosing an antenna that won't make my transmitter go boom
« Reply #24 on: October 29, 2016, 0157 UTC »
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

Offline ChrisSmolinski

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Re: Advice on choosing an antenna that won't make my transmitter go boom
« Reply #25 on: October 29, 2016, 1538 UTC »

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
Chris Smolinski
Westminster, MD
eQSLs appreciated! csmolinski@blackcatsystems.com
NRD 545 / netSDR / AFE822x / AirSpy HF+ / KiwiSDR / 670 ft horizontal loop / 500 ft northeast beverage / 58 ft T2FD / 300 ft south beverage / 43m / 20m / 10m  dipoles / Crossed Parallel Loop

Offline Stretchyman

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Re: Advice on choosing an antenna that won't make my transmitter go boom
« Reply #26 on: October 30, 2016, 1146 UTC »
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.
'It's better to give than receive' so why RX when you can TX!

                            Buy one from me, NOW!

Great discounts on ALL my transmitters if purchased via HFUnderground


                                              ;)

Offline Zazzle

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Re: Advice on choosing an antenna that won't make my transmitter go boom
« Reply #27 on: November 01, 2016, 1118 UTC »
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]
Trans-/Receivers: JRC NRD-525,  ICOM IC-R72,  YAESU VR-5000,  YAESU FT-897D
Antennas: 80M Halfwave Dipole,  40m Inverted-V,  5/8λ Groundplane,  20M Longwire,  misc. UHF/VHF Scanner Antennas.

Offline PirateSWL

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Re: Advice on choosing an antenna that won't make my transmitter go boom
« Reply #28 on: November 01, 2016, 1334 UTC »

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
Brian D. - PirateSWL
Pirate Radio Shortwave Enthusiast from NY
RX: Kenwood TS-480 / 40m dipole
Not embarrassed to admit I often use Shazam ; )
eQSL greatly appreciated to PirateSWL@aol.com

Offline Stretchyman

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Re: Advice on choosing an antenna that won't make my transmitter go boom
« Reply #29 on: November 02, 2016, 1732 UTC »
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.
'It's better to give than receive' so why RX when you can TX!

                            Buy one from me, NOW!

Great discounts on ALL my transmitters if purchased via HFUnderground


                                              ;)