New project - compact 20m antenna - no HV caps used :-) Good results from my apartment

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Re: New project - compact 20m antenna - no HV caps used :-) Good results from my apartment

Post by Admin on Tue Jul 25, 2017 9:09 pm

Hi Densil,
So glad that you have a success with the antenna.

1/4 of a Watt dissipation in 100 Watts is a very small loss. Those block connectors are designed to withstand high voltages, but only at low frequencies. At HF the dielectric may conduct a little and therefore disspiate power. As you are probably aware, capacitors do not dissipate power, so there must be some leakage at RF.

It is interesting that mains cable was used as the original capacitor, but at about 30 Watts or so the heat was sufficient to cause the plastic insulation to carbonise at the point(s) of contact. It worked ok for the 5 Watt QRP rig for which I originally made the antenna.

The antenna impedance matching at 14 MHz relies on the fact that the antenna has an end-impedance of about 3000Ω, which will give a near perfect match at 14MHz. At 7MHz you should aim for a slightly impedance, because the turns-ratio is greater if you are to have a 6-turn loop and a 1/2-turn feed loop. If you simply added more capacitance then the end-impedance would fall, the efficience would fall and the feed-loop would have to be increased to perhaps 1 complete turn.

When all is said and done, the efficiency of an antenna is still dependant on the amount of metal you put in the air, relative to the wavelength.

My design is proven since about 12 years ago, and this new-build has taken the frequency up, from 3.5 MHz to 14 MHz. If you make changes to the impedance then the match will get worse. Experiment by all means, and share your results with us here. I am sure that there are many members and guests who would be interested. Other people have played with my design and had some very interesting results and performance improvements.

Very best regards from Harry - SM0VPO

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Re: New project - compact 20m antenna - no HV caps used :-) Good results from my apartment

Post by Densil on Mon Jul 24, 2017 9:35 pm

Hi harry,
Ive been busy working on your 20m antenn. it worked perfectly as described but i used the cu board hv capacior you gave the picture of. i loaded it to 100w cw and it worked great but after about 2mins of tuning i noticed that the tuning cap was great but the connection block you used gets a bit warm. not enough to melt the plastic or anything yet there must be a bit of power wasted there. 1/4 a watt??

swr is 1-1 and the 20m band is alive with contacts. no detuning even with the slightly warm plastic block. frequ is rock steady at 14.2mhz. what do u think is the max power i can run with it?? my first contact was an sv station and then an ik. do u think it will work at 7mhz with just a bigger cap?? maybe i should do what you do - suck it and see Smile
73 de /D

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Re: New project - compact 20m antenna - no HV caps used :-) Good results from my apartment

Post by Admin on Wed Jul 19, 2017 7:47 pm

Hello Densil,
Nice to hear from you. Yes, I used to work with the Biconical Monopole antenna when I worked in GRSC in the Royal Air Force.

> good work harry
Thank you. Many of my projects are designed using more of a feeling than calculation. But that is what comes of experience (or is it old age?). Most of the time it is just knowing how to do it.

So what have you been doing? Have you had many contacts with the 2-tube TX? I would be interested in to hear how it went as you used a more powerful final stage. Did the single LC antenna matching circuit work the same?

Best regards from Harry
PS - What is your callsign?

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Re: New project - compact 20m antenna - no HV caps used :-) Good results from my apartment

Post by Densil on Tue Jul 18, 2017 7:50 pm

hi Harry, i like the suck-it-and-see technology. no formulas. nothing. many new ideas come from that.

uhf aircraft ground to air use a bicinical-monopole antenn that was built the same way and now used at airports worldwide. they just took 2 metal round things and hacked off bits until they got the radiation pattern an 3 to 1 frequency range wanted     The antenn is several wavelengths tall and it works.

good work Harry

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Re: New project - compact 20m antenna - no HV caps used :-) Good results from my apartment

Post by Admin on Mon Jul 17, 2017 10:30 pm

Hello Radiola,
To be perfectly honnest, my design for this new antenna was "suck-it-and-see", that is I made the antenna with a good guess, then measured it to see what it did. I then added or removed turns to get in the right area, then adjusted the turns space to give me resonance at about 500kHz above the desired frequency.

For 7MHz you should need about 6 turns, but each turn also adds capacitance. Try duplicating my 20m version, then add one extra turn inside, and two  extra turn outside, using the same turn spacing. This will make the antenna about 12cm bigger (6cm longer in every arm). Then check the resonance using a GDO, or a general coverage receiver and listen for maximum noise.

Without any capacitance you need to aim for max noise at about 6.7 to 7MHz. Add or remove turns to get this. The outer turn will give you more inductance than the inner turn. The natural frequency without the capacitor should be about 10% higher than the wanted frequency.

When you have done this you use the 10KV capacitor I described using two copper-clad boards sandwiched in dry plastic foam. Squeeze it (compress the foam sponge) to lower the frequency.

If you use this procedure then you will get a near-perfect match to 50 Ohms at the frequency you want.

Have I answered your question?

In the late 60s I used "Echo-Charlie" on 6.5MHz to 6.7MHz (45.5m). Is it the same band but shifted to 6.25MHz (48m)? In those days we used AM, but I guess you use SSB today? Indoors or outside? I ask because there are better antennas you can use for outdoor use, but this antenna does give you very good TX filtering and strong rejection of harmonics and other out-of-band spurious responses.

Best regards from Harry - SM0VPO
(Not yet QRV from the new QTH, but working on it)

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Re: New project - compact 20m antenna - no HV caps used :-) Good results from my apartment

Post by radiola on Wed Jul 12, 2017 10:26 pm

Hello!
I am interested in trying a loop antenna for pirate broadcasting on 48 meters pirate band.
Do you have some advice on how to calculate a suitable antenna?

radiola
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Re: New project - compact 20m antenna - no HV caps used :-) Good results from my apartment

Post by Ivan on Tue May 30, 2017 5:45 am

Hi Harry,
no, I do not have Echolink.
I hope there will not be much span on the forum. If that happens, you can always make the rules more rigid.

VBR from Ivan

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Re: New project - compact 20m antenna - no HV caps used :-) Good results from my apartment

Post by Admin on Mon May 29, 2017 4:18 pm

No problems Ivan.
It would be really nice to make contact with you someday and I was hoping that you may have had HF. But not to worry. Do you have Echolink?

Tomorrow I will be on 20m from work with a mobile station. But I will be pushing 50 Watts into a 1/4-wave groundplane antenna. I will see if I can get a photo.

By the way, what do you think of the new rules to allow free guest access? I hated the SPAM situatuion a few years ago, but now I think the problem has been solved with selecting the standing rhino and no SPAM links allowed.

Very best regards from Harry - SM0VPO

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Re: New project - compact 20m antenna - no HV caps used :-) Good results from my apartment

Post by Ivan on Mon May 29, 2017 1:27 pm

Hi Harry,
I feel myself more as a technician. I am not QRV on HF now, sorry.

VBR from Ivan

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Update

Post by Admin on Sun May 28, 2017 7:45 pm

This weekend was interesting. A good friend (ZL1TWA) sent me a couple of those old-fashioned dynamic microphones, and suddenly I get no RF into the MIC of either my FT101ZD (battery powered) or the X1M (also battery powered). No more Electret TRF mics.

The first thing, using battery > radio > antenna I get a rather high background wideband noise. Simply grounding the battery negative cured that.

The second thing, the 20m loop antenna had a span of 90kHz (VSWR = 1.5:1) with 14.175MHz centre frequency (VSWR about 1.15:1). I removed the balun and now the VSWR is 1:1 at from 14.17 to 14.21. VSWR 1.5:1 occurs at 14.11MHz to 14.23MHz (120kHz span). No difference in interference to telephones or TV Very Happy

The third thing is that I went over the road to the park and used a 5.4 metre vertical antenna in a metal umbrella mount. No radials needed to get close to 1:1 VSWR from 14.05MHz to 14.33MHz. I even heard a 5/9 JA station, and a pile-up for a Lebanese station, but my 5-Watts was no match for the "big boys". The propogation was very good yesterday (Saturday).

From my balcony I tried a full 50-Watts CW and the gimick capacitor on the antenna burned. But changing the capacitor to two bits of copper 5x7cm spaced 4cm apart I got resonance at center of band and no smoke using the full output of my FT101ZD.

I will update the project on the homepages with the changes.

But on a more interesting note; I plan to operate on 14.19MHz on Tuesday this week from the car-park at work, weather permitting. Using 50-Watts SSB. If I have the time I will be on every day of the week. If I stay in the department then someone always drags me into a meeting, os pushes a problem up my vest. It the WX is OK then I hope to operate every weekday during the summer, except Thursdays. So if either of you two have an HF rig then perhaps we can hook up on Tuesday? About 11:10 Stockholm time (GMT +2).

Very best regards Harry
@ Ruud - So glad you had a nice birthday. I will reply to your mail shortly but it was very interesting reading.
@ Ivan - Are you still active on HF? I worked RA3 and G4 from the balcony with 5W last week :-)


Last edited by Admin on Sun May 28, 2017 7:50 pm; edited 2 times in total (Reason for editing : seemed like a good idea at the time)

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New project - compact 20m antenna - no HV caps used :-) Good results from my apartment

Post by Admin on Tue Apr 11, 2017 7:30 pm

Hi everyone (and that means all three of you),

This Sunday I was still playing around trying to get a signal out on 20 metres, and I kept looking at my old frame antenna project. Problem is that I have no HV tuning caps, and PCB etching is out of the question (Maj-Lis is here in Sweden with me ;-).

So I wound come wire on some plastic bits, experimented with spacing and made a nice little antenna that does not need any tuning caps. I cab simply poike it out of the balcony window in a brush-handle. On first trial I got a contact in Ukrain with my little 5-Watt X1M. I could also hear L&C Denmark and France.

The project was inspired by others who took my 80 metre frame antenna and modified it for 20 metres, but mainly motivated by having a rig here without any means of using it in the apartment.

Total cost to build was SEK 18 (US$ 1.5) for a strip of block connectors, but only 1/3 of the strip was used. Does that make it a $1/2 antenna?

Anyway, I was surprised by the results. Even better than I though it would be. VSWR is almost perfect at 14.175MHz.

Hope someone out there finds it and puts the project to good use. It took me longer to document it that it took to make it. But it was also a maiden voyage for my GDO-2 in it's first serious application (setting off the theft alarms in H&M is not considered serious ;-)

Very best regards from Harry - SM0VPO

PS - I wonder - how about a multiband loop antenna?

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