- January 4, 2017 at 5:54 pm #11041RichPowersParticipant
Total posts : 421
I’m trying to grasp exactly what Ermi Roos was sayinging in the following post from back in 2008. It’s bugging the crap out of me..
It’s dancing right there in front of my eyes but I just have not been able to grab it.. Can anyone interprit this in toddler terms?
(This is copied from http://www.part15.us/blogs/ermi-roos/earliest-radio?page=1 beginning at the end of Post 42 and then continues in Post 44)
I will next show that the signal does not have to be as low as the Section 15.209 range if a ground lead is not used. A 3 m dipole remote from ground has an input impedance of about .05 – j6000 ohms. It would not be unreasonable for me to assume the same 25 ohms loss resistance that I assumed for the monopole over ground, because, although the loading coil inductance has to be doubled, there is no ground loss. The radiated power is 50 uW. The field strength at one radianlength is 1.69 mV/m, taking into account the lower gain of a dipole compared to a monopole. If a vertical dipole is not remote from ground, the radiation resistance increases, increasing to .1 ohms very close to ground. An elevated vertical dipole launches both a skywave and a groundwave. If the lower arm of the dipole has a ground lead, the radiation resistance goes up to .2 ohms (excluding the loss resistance which now includes ground resistance), and the antenna reactance goes down to around the midpoint of 3000 and 6000 ohms. Only groundwave is launched. A grounded 3 m dipole gives better performance than a 3 m monopole above ground.
What I have illustrated here is that a Part 15.219 transmitter produces radiation down at the Part 15.209 levels, as the FCC originally intended, only if it is specifically designed for poor efficiency, such as transmitters intended only for operation inside of a home. Part 15 AM transmitters tend to work very well in the home because of the high near field ar very short distances. A ground lead, even if it is not intended to be radiating, helps efficiency tremendously. However, even if a ground lead is not used, 15.219 transmitters can produce much higher field strengths than the 15.209 limits.
A vertical dipole not remote from the earth, but elevated above ground far enough so that its input impedance is nearly the free-space value, launches a skywave that eventually follows the surface of the earth. This wave is similar to, but not exactly the same as, a groundwave, so it is called a quasi-groundwave. At a distant point, the wavefront of the quasi-groundwave is the combination of two different waves. One wave is reflected, and the other is direct. A small vertical monopole above ground, and a short grounded dipole, both generate only groundwaves. At a distant point, only a single wave is present.
This distinction between the quasi-groundwave and the groundwave makes a difference in the strength of the signal received at a distant point because of either constructive or destructive interference between the two vaves of the quasi-groundwave. No interference occurs due to a pure groundwave, because only a single wave is present.
Edit: Maybe there is a confusion about the meaning of the words “skywave” and “groundwave.” A skywave originates from an elevated direction, and it can be reflected from the ground plane. A ground wave cannot be reflected from a perfect, flat, ground plane because the wavefront propagates along the surface of the ground plane. The groundwave can be reflected from an elevated reflector, like the ionosphere. The reflected groundwave becomes a skywave.
The above edit is his not mineJanuary 5, 2017 at 9:14 pm #52794Nate CrimeGuest
Total posts : 45366
It reads to me that the RF is being fed to a vertical system above ground, and there’s a top section of antenna, a split where the RF is applied, and the bottom part of the antenna goes to the ground and connects to it. With the RF fed to the split, the antenna acts like a dipole, even with one side grounded or unequal lengths on each side.
I wish the site had more people like that these days, thinking about theories and possibilities rather than rules and restrictions and all the “new woes”. I’m a nerd for that kind of tech stuff, and the comparison to traveler’s service stations that have a hundred times the ERP of what we’re supposed to have, from the original article.January 6, 2017 at 7:38 am #52799RichPowersGuest
Total posts : 45366
I found the key!
I removed all the text that confused me, and what remained revealed the answers…
I will next show that the signal does not have to be as low as the Section 15.209 range if a ground lead is not used. A grounded 3 m dipole gives better performance than a 3 m monopole above ground.
What I have illustrated here is that a Part 15.219 transmitter produces radiation down at the Part 15.209 levels, as the FCC originally intended, only if it is specifically designed for poor efficiency, such as transmitters intended only for operation inside of a home.
A ground lead, even if it is not intended to be radiating, helps efficiency tremendously. However, even if a ground lead is not used, 15.219 transmitters can produce much higher field strengths than the 15.209 limits.
Ok.. So, A grounded transmitter works better than one without ground, and 15.219 produce higher field strength than 15.209
Well I already knew that.
At least the quest was fun.January 6, 2017 at 6:28 pm #52806wdcxGuest
Total posts : 45366
I miss what Ermi brought to the table.
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