[TowerTalk] Cushcraft/MFJ Traps

[jimlux]

On 9/13/19 6:17 AM, john at kk9a.com wrote:
> 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.


I think the OB2-40 has 10 meter long elements, which is about 50% of the 
full size resonant dipole for 40 meters.
Your modeled 2dB change is fully consistent with the calculation below..

As someone else pointed out, you do potentially give up some gain from a 
physically shorter antenna - or a narrower bandwidth, or something else. 
(TANSTAAFL)

There's even an equation for it: The Chu-Harrington formula tells you 
the tradeoff between physical size, Q, directivity

https://en.wikipedia.org/wiki/Chu%E2%80%93Harrington_limit



> 
> 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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