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Author Topic: 630M Dipole????  (Read 658 times)

KT4WO

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630M Dipole????
« on: February 14, 2020, 05:43:23 PM »

Has anyone tried a halfwave dipole on 630??

I have the wire(1000' 14ga stranded copper) and the room.
It will be low...20-30' high end to end(if I can find enuff supports)

Seems everyone is using verts. or other short antennas.

Thoughts?

Trip - KT4WO
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VE1YY

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Re: 630M Dipole????
« Reply #1 on: February 14, 2020, 06:46:12 PM »

Good day, Trip.
I presume you enquire about a horizontal dipole. Such a low dipole will be a grand cloud warmer! You, nevertheless, might want to try it for learning purposes, ours and yours.

Verticals are used on 630m in order to achieve good ground wave propagation and good ionospheric propagation (D and E region).

You would benefit from making a vertical 20 to 30 feet high (better higher). Attach the wire to the tip of the vertical as a form of top loading. One or more wires going off in opposite directions works. The antenna will need less base or middle inductive loading to achieve resonance with such top loading.

Whatever, you do, remember that verticals need very good ground systems.

I am using a 28 foot high by 100 foot wide Marconi T Vertical over (40) 64 feet long radials and about 1 watt EIRP. Two-way QSOs of up to 3000 miles are in the log.

73 and GL,

Bill
VE1YY
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VK6HP

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Re: 630M Dipole????
« Reply #2 on: February 14, 2020, 06:55:26 PM »

Trip,

Sounds like you're better equipped real-estate wise than many of us on 630 m.  Perhaps you might find the LF antenna pages of ON7YD interesting, and especially the diagrams of the low horizontal dipole:

 http://www.strobbe.eu/on7yd/136ant/#HorDipole

As you can see, even the full size dipole has a very small radiation resistance and low efficiency. He's talking about LF rather than MF in the discussion but, despite some strongly frequency dependent terms, the same principles apply.

But the good news is that, with all that space and wire, you're in great shape.  I'd reserve a bit of space for at least one Beverage (very low) receiving antenna to get you started.  For transmitting, an L or T vertical is pretty simple.  I use my ZS6BKW HF doublet re-purposed as a Marconi T.  I am pretty space-limited but, nevertheless, am planning an antenna upgrade to complement a new 300 W PA (see my QRZ page if you're interested).  With the previous 70 W system I've had tans-continental two-way contacts on JT-9 and regular WSPR spots from New Zealand, even with the power reduced to 5 W.

What's your soil conductivity like?  Very likely the simple Marconi or similar system is a good start but if you have the room to support an elevated counterpoise (likely loaded), you could make an efficient system.  Caution: safety considerations apply in terms of RF voltages and currents.  Have a look at the "wall of flame"  pages if you need convincing :)

73, and good luck,
Peter.

« Last Edit: February 14, 2020, 07:03:05 PM by VK6HP »
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KT4WO

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Re: 630M Dipole????
« Reply #3 on: February 16, 2020, 04:29:30 PM »

Tnx Peter!

That link helped alot.  I am shocked tho. abt the dipole but looking at the
chart that explains it.
I have never been(TX) below 160M(where my dipole works GREAT)
so I guess I will think abt some other antenna when I get my txverter.

BTW..I don't have 1000+ feet of my own property(abt 650ft is mine) but
other owners don't mind.(it's forest)

Tnx agn de Trip
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KM1H

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Re: 630M Dipole????
« Reply #4 on: February 17, 2020, 09:40:54 AM »

With accessible trees available a vertical top loaded T will radiate some RF. Use a loading coil in the vertical portion and a nano VNA to determine the matching required.
With a CW beacon Ive been heard in Sweden with ~ 5W ERP before it became a ham band here.

No time to take it to the next step.

Carl
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RFRY

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Re: 630M Dipole????
« Reply #5 on: February 23, 2020, 05:02:07 AM »

...I have the wire(1000' 14ga stranded copper) and the room.  It will be low...20-30' high end to end(if I can find enuff supports).  Seems everyone is using verts. or other short antennas.

Below is an example of a 475 kHz v-pol antenna setup that might fit your installation site:

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W1VT

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Re: 630M Dipole????
« Reply #6 on: February 23, 2020, 07:41:09 AM »

Low band antennas are compromises to fit on the land you have available.
I tried a top loaded T on 160 but it was too noisy as it needed to run out near the power lines.
An inverted L fits in my back yard and is significantly quieter, so sometimes it works great on receive.

You may want to just figure out what supports you have, string up some wire, and then figure out how to use it.
Much better than doing a theoretical antenna design and finding out that no matter what you do, it just isn't going to happen that way.

Zak W1VT
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KM1H

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Re: 630M Dipole????
« Reply #7 on: February 23, 2020, 08:43:59 AM »

Quote
Low band antennas are compromises to fit on the land you have available.
I tried a top loaded T on 160 but it was too noisy as it needed to run out near the power lines.
An inverted L fits in my back yard and is significantly quieter, so sometimes it works great on receive.

There is a lot of experimenting needed on 630 especially if a separate RX antenna is not used.

A nice thing about the T is that it suppresses the straight up horizontal cloud warmer lobe and minimizes the variations in the vertical pattern lobes.

Ive used 500 to 750' Beverages for RX and strangely they show good directivity even down to 100 kHz or so. Another effective RX antenna is a rotatable balanced loop fed into a balanced preamp and the output coax back to the house. The preamp can be remotely varactor tuned and rotated. I use two turns 1" CATV hardline. Tuning can be quite sharp which does wonders to reduce noise pickup.
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VK6HP

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Re: 630M Dipole????
« Reply #8 on: February 23, 2020, 06:30:11 PM »

If you can get away with using the same antenna for transmit and receive on MF and LF, count yourself lucky.  There are some very successful operators who manage it but they are often based in more remote, RFI quiet locations.  Even then, as Carl implies, it's useful to have a few receive options, perhaps including a loop or Beverage, if only to account for propagation variability.  For most of us working from more urban locations separate antennas are usually the order of the day.  Nevertheless, most of us do set up the T/R switching etc so that we can listen on the transmit antenna if desired.

While there are a lot of local variables, I find modelling of the type shown by Richard to be extremely valuable, if only as a way of underlining some of the inconvenient truths that often get lost in anecdote and in the face of desperation.  There's a great deal to be said for getting into the ballpark with a given design, especially at these low frequencies where structures are large and the effort in getting something erected and working can be significant.

With a bit of space and wire available, Trip (OP) could do worse than give the elevated T a go. I'd probably be tempted to increase the height of the vertical section a bit but, regardless, it'd be a good place to start.

73, Peter.
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KH6AQ

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Re: 630M Dipole????
« Reply #9 on: February 24, 2020, 12:13:40 PM »

Has anyone tried a halfwave dipole on 630??

I have the wire(1000' 14ga stranded copper) and the room.
It will be low...20-30' high end to end(if I can find enuff supports)

Seems everyone is using verts. or other short antennas.

Thoughts?

Trip - KT4WO

The FCC allows 5 watts EIRP or 500 watts RF output, whichever is less. I'll post that comparison next.

Let's compare a half wavelength dipole 25' above 0.005/13 GND against a 30' vertical. Note that the dipole exhibits maximum low angle gain off the ends.

Dipole, 25' above ground
Radiation efficiency = 8%

Vert, 30' with 4 x 22' sloping top wires, Rr = 0.14 ohms, near-field radiation efficiency = 1.0%
917uH, Q = 200 loading coil
12 x 30' radials 0.1' above GND
Allowable RF power = 500 watts

ELEVATION (deg)    GAIN (dBi)    FS RATIO (dB)   VERT GAIN (dBi)
5                             -18                       8                      -24
10                           -15                       6                      -21       
20                           -12                       3                      -19
30                           -11                       1                      -19
45                            -9                        0                      -20

« Last Edit: February 24, 2020, 12:38:18 PM by KH6AQ »
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KH6AQ

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Re: 630M Dipole????
« Reply #10 on: February 24, 2020, 01:12:36 PM »

Dipole vs vertical with FCC maximum 500W/5W EIRP
(the vertical is the winner)

I'm a little fuzzy on this so please correct and teach as needed. I believe EIRP is figured using the maximum antenna gain in the far- field. If so, we use the maximum antenna gain which is straight up for the dipole and at 21 degrees for the vertical. The dipole maximum gain is -6.3 dBi while the vertical is -17.3 dBi. Within the confines of the FCC 500 watt max RF input or 5 watt EIRP limits this says the dipole can be driven with a maximum of 21 watts and the vertical can be driven with 269 watts, or 11.1 dB more than the dipole. At an elevation angle of 30 degrees the dipole gain is -12.3 dBi and the vertical gain is -17.6 dBi. While the dipole exhibits 5.3 dB more gain at 30 degrees the vertical can be driven with 11.1 dB more RF power giving it 5.8 dB more signal at 30 degrees.
« Last Edit: February 24, 2020, 01:26:09 PM by KH6AQ »
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RFRY

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Re: 630M Dipole????
« Reply #11 on: February 24, 2020, 01:20:45 PM »

...Let's compare a half wavelength dipole 25' above 0.005/13 GND against a 30' vertical. Note that the dipole exhibits maximum low angle gain off the ends.

NEC4.2 is showing maximum radiation perpendicular to the length of the dipole (below).

Have you investigated/compared the radiated fields from your dipole and your vertical including the surface wave for elevations ranging from zero to about 15° at a distance of 50 to 100 wavelengths from the radiators?


« Last Edit: February 24, 2020, 01:39:05 PM by RFRY »
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KH6AQ

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Re: 630M Dipole????
« Reply #12 on: February 24, 2020, 01:38:16 PM »

I believe the gains reported are calculated by NEC-2D are figured beyond 100 wavelengths.

What dipole azimuth pattern does NEC4.2 show at 10 degrees elevation?

If my numbers are correct one can meet the 5 watt EIRP FCC limit using a modest vertical driven with less than the 500 watt FCC max RF power.

I've obtained permission to operate on the 630 meter band and now it's time to build the antenna and build or buy a 250 watt amp. I haven't checked it myself, but the IC-7300 is supposed to output a couple hundred milliwatts at 475 kHz. I have a 34' vertical with 32 radials (just about like the simulation) and will add top hat wires and a loading coil.

Here are 630 meter amps for sale by K5DNL       http://www.k5dnl.com/sale_amp150.htm

« Last Edit: February 24, 2020, 01:53:18 PM by KH6AQ »
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RFRY

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Re: 630M Dipole????
« Reply #13 on: February 24, 2020, 01:56:16 PM »

... If my numbers are correct one can meet the 5 watt EIRP FCC limit using a modest vertical driven with less than the 500 watt FCC max RF power. ...

It seems likely that when evaluating the radiated fields of these antennas so as to meet an FCC EIRP limit using only a NEC far-field calculation that higher fields are being unreported, and may exceed the fields that would be generated by the FCC EIRP limit at distances fairly close to the radiator.
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W9IQ

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Re: 630M Dipole????
« Reply #14 on: February 24, 2020, 02:02:10 PM »

I'm a little fuzzy on this so please correct and teach as needed. I believe EIRP is figured using the maximum antenna gain in the far- field. If so, we use the maximum antenna gain which is straight up for the dipole and at 21 degrees for the vertical. The dipole maximum gain is -6.3 dBi while the vertical is -17.3 dBi. Within the confines of the FCC 500 watt max RF input or 5 watt EIRP limits this says the dipole can be driven with a maximum of 21 watts and the vertical can be driven with 269 watts, or 11.1 dB more than the dipole. At an elevation angle of 30 degrees the dipole gain is -12.3 dBi and the vertical gain is -17.6 dBi. While the dipole exhibits 5.3 dB more gain at 30 degrees the vertical can be driven with 11.1 dB more RF power giving it 5.8 dB more signal at 30 degrees.

The FCC working definition of EIRP is:

     EIRP = PT+GT-LC

     where PT is the transmitter power in dBW, GTis the antenna gain in dBi, and LC is the cable and matching circuit losses in dB

The 5 watt EIRP power limit for 630 meters expressed in dBW form is 7 dBW. Neglected cable and matching circuit losses, the maximum transmitter power for the dipole example is therefore 7 dBW - (-6.3 dBi) = 13.3 dBW or ~21 watts.

For the vertical example, again ignoring cable and matching circuit losses, the maximum power is 24.3 dBW or 270 watts.

Note that both antennas will perform equally well (in their respective maximum gain direction) when you use this technique. So other than the wasted power, neither antenna has a transmit field strength advantage.

Also note that in practice, the losses of the matching networks can be substantial so this will further raise the maximum power.

- Glenn W9IQ
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- Glenn W9IQ

God runs electromagnetics on Monday, Wednesday and Friday by the wave theory and the devil runs it on Tuesday, Thursday and Saturday by the Quantum theory.
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