Posted by
Lyn WØLEN on
Jul 18, 2020; 1:18am
URL: http://elecraft.85.s1.nabble.com/Matching-resonant-antennas-tp7663149p7663325.html
Barry -
+1
I use nothing but 600 ohm OWL (True Ladder Line) and a short piece of coax connects to a 1:1 / 4:1 hybrid balun to allow matching the impedance perfectly with my KAT500.
The antenna is a 360' center fed EDZ (design frequency of 3.5 MHz). The KAT500 matches it on all bands 160 - 6m (on 15m, it bypasses).
The measured performance indicates excellent radiation on all bands.
73
Lyn, W0LEN
-----Original Message-----
From:
[hidden email] [mailto:
[hidden email]] On Behalf Of Barry LaZar
Sent: Friday, July 17, 2020 7:44 PM
To:
[hidden email]
Subject: Re: [Elecraft] Matching resonant antennas
Wes,
You're correct that open wire/ladder line transmission lines are
not a panacea. But, in the average ham stations, open wire does overcome
high losses with high SWR, or not so high SWR. What is needed is a look
at the ARRL Antenna book for transmission line loss/100 ft. as a
function of SWR. You see that on 10 meters running 10:1 SWR the total
loss is around 1 db. And, as you go to the lower bands, losses become
less. Typical 400 Ohm ladder line has a loss of 0.2 db at 10 MHz and 0.6
db at 100 MHz. Using these data and a little interpolation, I would use
0.4 db on 10 meters and a 10:1 SWR for this to be 0.8 db of additional
loss for a total of about 1.2 db. Yes, I do use a balun and recommend
them so add another 0.5 db. Add another 0.5 db for a good tuner and we
end up with a total of 2.2 db. on 10 meters and less on 20 and it
decomposes to an academic exercise on 40 and down.
Coax on 10 starts out with a higher loss/100 feet. I will use what
I use here in K3NDM, Times LMR400. That represents 0.4 at 10 MHz. and
1.4 db at 100 MHz. That will yield about 1.2 db/100 ft on 10 meters. Add
0.25 db for a 2:1 SWR and 0.5 for tuner loss and you end up with about
1.95 at a 2:1 SWR.
Here coax wins IFF the SWR remains less than 2:1 which won't happen
over the entire commonly used portion of the band. Using ladder line
vice coax will contain losses over the entire band. This keeps things
simple and the cost should be a lot less.
As to the RCA station with a 14:1 SWR, they used no balun nor tuner
as we commonly know them today. The coupling to the final tube(s) were
balanced and was capable of making the transformation from what the tube
wanted and the reflected impedance at the transmitter end of the
transmission line. Ergo, they had a very low loss if using 10 meters
which they didn't. They typically used frequencies below 18 MHz and a
slug of power.
Vy 73,
Barry
K3NDM
On 7/17/2020 6:57 PM, Wes wrote:
> I wrote about some of this in my paper ARRL Antenna Compendium paper
> on ladder line 20 years ago.
https://sadxa.org/n7ws/Ladder_Line.pdf>
> I've seen pictures of those SW transmitting plants and always assumed
> that they must have been very efficient. Upon reflection---no pun
> intended---now I'm not so sure. If losses mounted up, (which they
> most certainly did at 14:1 SWR) they had the option of just turning up
> the wick.
>
> Furthermore, as I said in closing: "Contrary to the conventional
> wisdom, ladder line is not a panacea for every transmission line
> problem." In the ensuing 20 years, I've become even more convinced of
> this. When tuner and balun losses are factored into this picture I
> don't know why anyone would want to use this stuff. I sure don't.
>
> I have a KAT500 and use it to tune some way-off resonant antennas (fed
> with 7/8" Heliax) but I don't delude myself into thinking "I've
> contained system losses."
>
> Wes N7WS
>
https://www.qrz.com/db/N7WS>
>
> On 7/17/2020 2:32 PM, Barry LaZar wrote:
>> Resonance is over rated. The problem of believing you must have a
>> resonant antenna arose with the use of coax cable began. High SWRs
>> causes high system losses.
>>
>> Prior to the widespread use of coax, open wire was used and few
>> antenna systems were really resonant, and nor were they reflecting a
>> 1:1 SWR. Back then, no one cared as tubes were used and pi-net or
>> swinging links were used to match to whatever was connected to the
>> transmitter. In fact, I once visited a site that used rhombic
>> antennas and Sterba curtains being fed by high power transmitters.
>> The feed line were copper pipes about 1/4" in diameter and spaced
>> about 4". The SWR, I was told, was 14:1. I asked if that was a
>> problem of transferring energy to the system. The answer was no as
>> the final output stage could match it and the system losses were low
>> due to the type of feed line used. This was a lesson I learned 60
>> years ago and haven't forgotten it. The site was the RCA site the
>> once stood on Montauck Point on Long Island, New York.
>>
>> One point that keeps getting forgotten is the conservation of energy
>> concept. What that means is energy can only be changed and not lost.
>> Typically that means transmitter energy would be changed to heat, but
>> not lost. What is not changed to heat on the coax will make it to the
>> antenna where it MUST be radiated and not lost. Yhe practical
>> application of this is use really good coax if you can't get to a
>> 1:1-2:1 SWR, ot there about. Alternatively, use ladder line and a
>> current balun. Elecraft tuners easily tune 10:1 SWR which contains
>> system losses nicely. I have been doing this for a very long time and
>> have achieved WAS, DXCC phone, DXCC CW, and DXCC digital, and, I'm 13
>> short on 80 of making 5BDXCC.
>>
>> 73,
>>
>> Barry
>>
>> K3NDM
>
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