Much the same, I had the Mosley shorty forty for years. After I took down my 3
element Wilson it worked well for me, however the BC climate played havoc with
the coils with green slime having to be removed occasionally.
Glenn, VA7UO
Sent from my iPhone
> On Sep 13, 2019, at 6:17 AM, john@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.
>
> 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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