[Participant]

Anonymous dňa January 31, 2009 at 12:21 #11132

What antenna to use under these conditions for 160m and 80m?

The dimensions are approximate.
Red are metal parts of the construction and grounding.
The machine rooms with a crown above the roof have a height of about 3m, the width of the crown is about 1m. On one machine room, there is about a 5m mast with shared TV antennas.
The antenna should be placed only on the roof, so no sloper etc.
I originally thought of W4D but some comments regarding TVI discouraged me (the landlord wouldn't tolerate them) now I'm considering G5RV, but it is optimized for 14MHz.
What antenna should I choose?

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om3cvv dňa February 1, 2009 at 13:44 #12426

The dipole is the least disruptive. Do you want that antenna for transmitting, receiving, or for playing?

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[Participant]

om0aao (OM0A) dňa February 1, 2009 at 16:43 #12427

Excellent drawing. It's a pity to have such a roof for some horizontally stretched antenna for 160 and 80m. A directional antenna, say HB9CV, would fit there for higher bands. That would work really well.

A horizontal dipole-type antenna – and all those mentioned – should be stretched high above the roof. Otherwise, its properties will be degraded by the roof and due to the small distance, there will be a significant induction into the apartment building.

Try in some program, e.g., MMANA, what moving the antenna from free space to real space does and you will get an answer.

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[Participant]

Anonymous dňa February 1, 2009 at 20:20 #12428

to om3cvv: for transmission and reception

to om0aao: I tried to simulate something in MMANA but it still evaluated the reactive component of the impedance as incredibly large even for inv L. I'm also considering antennas for 21 and 28, but I would like 80m as a priority band. Unfortunately, I can't come up with anything satisfactory. That crown creates an incredibly high ground.

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Anonymous dňa February 1, 2009 at 21:35 #12429

You don't have much to think about. For 80m, stretch a full-wave loop around the perimeter about 2 to 2.5m above the structure, a thin, insulated wire is sufficient. (Insulated will be quieter, preventing static discharge). The length will shorten by a few meters compared to calculations due to insulation. Given the building, the loop will have slightly variable impedance depending on frequency, from 150 to 300 ohms. Otherwise, the loop will not interact with the ground (with the building). Connect via a current balun 1:4, coax feed (multiband options from 80 up will not spoil). Never tune the 80m loop for 160m !! Considering theusing a 1:4 balun, feel free to tune with a regular tuner, it doesn't have to be balanced. (Once again, coax feed). For 160m, stretch a 1/4 wave long wire, connect with a 1:1 balun and coax. The current balun must have a direct connection to the wire (radiator). Ground to the grounding of the apartment building and add about 11m of wire as a counterpoise (experiment with the length of the counterpoise PSW 2:0 you must have without a tuner). On the TX side, in the case of the 160m antenna use a coax choke (about 5.6 – 7m of coax – winding next to winding on a plastic form 8 to 15cm). Behind the radio (but before the tuner) or behind the linear (but before the tuner) inserta cheap low pass filter. You won't disturb anything ever. The loop will work with the magnetic component of the wave, not the electric one, it will be quieter than everything else. You will be plus 30 dB at home and considering the height you will be a decent DX. 1/4 on 160m (wire) will be decent DX, unfortunately a slightly smaller signal in the area.

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[Participant]

Anonymous dňa February 1, 2009 at 21:49 #12430

I still forgot. Such an elongated loop on 80m will tend to behave like a folded dipole. So, if you are going to measure bare wires with an analyzer, look for an impedance of 200 ohms, as with a folded dipole. Don't look for slucky 100 ohms, I'll hardly ever be there. Measure with an analyzer through a 1:1 balun intended for measurement, or a standard one. The loop is easy to model, use 4NEC2X (Demo) it will help you find the power point if you have a preferred direction for radiation. At this height, there will be a slight directionality. The loop is oriented horizontally, so the radiation polarization can ONLY be horizontal. Don't worry about DX with this polarization, in the US on the East coast you will be S7-S8.

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om3cvv dňa February 2, 2009 at 9:13 #12431

If you need that antenna for transmission, I see no reason why you shouldn't state your call sign.

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om0aao (OM0A) dňa February 2, 2009 at 9:46 #12432

@User wrote:

You will not disturb anything, ever.

As I recently found out, TVI can easily occur even on an imperfect TV system. With a poor SCART cable, interference occurred at 5W, with a quality one the TV is absolutely calm even at 700W power.

The proposed loop solution is good. An alternative could be a dual-band inverted L with elevated radials. It would work perfectly, but it would require at least a 10m tall vertical part and the ends of the radials would come close to the mast with the antennas.

Description http://www.160m.de/Invisible%20Antenna080406.pdf

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Anonymous dňa February 2, 2009 at 10:22 #12433

slucka.

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om0aao (OM0A) dňa February 2, 2009 at 11:51 #12434

I am sorry, but with 5W from the FT-1000MP, the harmonic content is so low that it simply cannot be a harmonic from VHF or UHF. The FT-1000MP uses seven low-pass filters, and at the end of the shack, I have a Kenwood LF-30A.

Another thing is if a signal of sufficient intensity gets to some nonlinear element in the TV, and it produces harmonics.

Inv.L s vyzdvihnutým radiálom cannot have a problem with the ground. Moreover, it offers a significant vertical component of the signal.

Slučka na 80-ku is not a bad solution for a city where a lot of interference can be expected. Moreover, it is a cheap solution that can be easily canceled if it does not suit.

Still, I think that a horizontal antenna for the lower bands belongs in space, not a few meters on an iron colossus. So smerovku na horné bandy na strechu a drôt (dipól, G5RV, LW, atď.) zo strechy na iný vysoký bod.

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[Participant]

Anonymous dňa February 2, 2009 at 12:45 #12435

In such conditions, an EH antenna can be a good solution.

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[Participant]

Anonymous dňa February 2, 2009 at 12:49 #12436

What do I know... Well, okay, an inverted L with an elevated radial will be great. Just a resonant quarter, for example, at 1.85 MHz will be 38.5 meters and the counterpoise (radial) will be 40.5 meters, and let's forget about the output to the grounding wire. Some call it a top-fed vertical (I don't know why) where the feed is asymmetric, some call it inverted L, and some call it a 'mangled dipole', which probably has the closest relation to that radial. I still recommend starting from the fact that inverted L works as such if it is fed against the ground. An inverted L is a cultural matter, it comes frommass use in England, where poverty and lack of space forced 70 thousand radio amateurs to build a stick in the corner of a mini garden and climb up it with a wire as high as possible (so the wire would shorten) since the chimney of the house is drastically close (therefore the horizontal part must be short). What else did they have left but to celebrate the massive vertical component of the antenna? (So they could even fall asleep). Plus to come up with incorrectly tuned coax traps and make a multiband. I am not saying it doesn't work, but it doesn't work well. Let's admire in England the Beatles, Led Zeppelin, and alsostrawberry jam Mrs. Darlington, but for God's sake, NOT antennas. In the field of art, brutal suffering led to beautiful artistic expressions. However, if any possibilities to apply a full-size manufactured antenna were suppressed, it did lead to extraordinary ingenuity, but unfortunately not to results.

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om0aao (OM0A) dňa February 2, 2009 at 1:09 PM #12437

I agree, although I don't know if inv.L originated in England. It is used there, however.

I based my work on materials from DK5WL. In the last CQWW SSB, I made over 460 QSO on 160m with this antenna using three(!) radials on the ground and 400W of power.

ON4UN did three times more on a 1/4 antenna, but not everyone has the opportunity to set up a 40m vertical to the house and run a few beverages. And it probably wasn't QRP 😀
http://lists.contesting.com/pipermail/3830/2008-November/162290.html

Very good results could be achieved with a quad in space.. Povedzme taký 3el. bidirekcionálny quad
http://web1.avanceit.com/F5VKM/Antenna%20Designs%20-%20160m%20Band%203%20Element%20Bi-Directional%20Delta%20Loop%20Beam.html

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om0aao (OM0A) dňa February 2, 2009 at 1:13 PM #12438

The EH antenna would certainly fit on the roof. In the category of super small antennas, it is definitely the best. Unfortunately, compared to full-size antennas, it lags by -10 to -12 dB, and that is noticeable.

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[Participant]

Anonymous dňa February 2, 2009 at 2:17 PM #12439

@OM0AAO wrote:

The EH antenna would certainly fit on the roof. In the category of super small antennas, it is definitely the best. Unfortunately, compared to full-size antennas, it lags by -10 to -12 dB, and that is noticeable.

I am coming from the assignment where it is clearly stated that 'The antenna should only be placed on the roof, so no slopers, etc.'
In that case, I forget about various loops, inverted Ls, etc. It is clear that it cannot compare to full-size antennas. But as an extreme solution, it is not bad. I even think it may perform better than various loops or wires that are just above the roof and not in open space. It needs to be tested.

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