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Re: [TowerTalk] Cushcraft/MFJ Traps

To: towertalk@contesting.com
Subject: Re: [TowerTalk] Cushcraft/MFJ Traps
From: john@kk9a.com
Date: Fri, 13 Sep 2019 08:17:10 -0500
List-post: <mailto:towertalk@contesting.com>
I currently have a pair of 2el shorty 40's made by Optibeam, model OB2-40. These use a high Q coil in the center of the elements for loading and have an 18' boom. For months I have been modeling various full size 40m Yagis and comparing them to my small antennas. Larger antennas have more bandwidth but I have been amazed at the efficiency (at least in my model) of OptiBeam's shortened elements. If I replace my current small 75 pound antennas with two full sized 350 pound 4 element OWA beams on a 48 ft boom, I will be only 2dB louder.

John KK9A

jimlux wrote:

On 9/12/19 12:42 PM, Jim Brown wrote:

I have two very different takes on this. First, traps are an inefficient way to build a multi-band antenna. That means they suck up gain. Also, because the elements are shorter, their radiation efficiency is reduced. The best multiband antennas don't use traps.

I'm not so sure about the efficiency aspect for shorter antennas - sure,
for "very short", the matching network losses will increase, but the
actual antenna efficiency isn't different (I^2*R losses are usually
pretty low)

Take a 6 meter long dipole as an example. REsonant at roughly 24 MHz -
here's the feedpoint impedance
f       r       x
23.6    77.6359 -12.4921
23.8    79.7763 -2.8992
24      81.976  6.6949
24.2    84.237 16.2932
24.4    86.5613 25.8988

Now let's drop to 18 MHz, so the dipole would be 75% of resonant length
f       r       x
17.6    33.6036 -316.66
17.8    34.6034 -305.433
18      35.6276 -294.325
18.2    36.6769 -283.331
18.4    37.7518 -272.445

So you'd need some sort of matching network to cancel out the 300 ohm
reactance. It's pretty easy to come up with a coil that has a Q of 200,
so the 300 ohm coil would have a resistance of 1.5 ohms. Compared to the
36 ohm radiation resistance, that's about 4% or 0.2 dB.

At 50% length:

11.8 12.9553 -732.472
12 13.4523 -713.564
12.2 13.9618 -695.14

Now we're starting to be significant, a inductor Q of 200 is going to be
around 3.5 ohms loss resistance, and against 13.5 ohms antenna R, that's
a 20% loss (1 dB).

Of course, for many HF links, on receive, the SNR is determined by the
atmospheric noise, and antenna gain (for lowish gain antennas < 10dB)
doesn't change the received SNR - the reduced gain drops both the
desired signal and the noise level.

For transmit, of course, it does affect the SNR that the other end sees.


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