[TowerTalk] Low loop in a sloping configuration

[David Gilbert]

Received signals don't just magically find their way to the feedline ... 
they induce a current in the antenna that behaves virtually the same as 
transmitted signals.  Incremental field radiation and incremental field 
reception both follow current distribution.  Your proposed loop puts a 
greater percentage of the current close to the ground ... thereby 
raising your takeoff and receive angle because of ground reflectivity.  
It's not quite a cloud warmer, but it's heading in that direction.

Think of it this way ... would you intentionally drop your Inverted-V 
from 90 feet to 45 feet?  No, and the reason you don't is the same 
reason you don't want to shift your current distribution closer to the 
ground.

With regards to your other comment, size generally doesn't increase 
signal-to-noise ratio.  It increases the strength of the captured signal 
(usually), but it also increases the strength of the noise unless you 
get significant directionality improvement out of the larger loop.  
Small loops can be effective RX antennas mostly because they can have 
sharp directionality for nulling unwanted noise/QRM.

But yes, large loops can be effective RX antennas if they point in the 
desired direction.  You proposed loop won't (in terms of takeoff angle), 
though, at least not compared with your Inverted-V.   At least half of 
the effective field radiation/reception in your proposed loop is going 
to be at a lower height above ground than the current in your Inverted-V 
... probably more than half since you're going to have to decrease the 
apex angle for your loop.

73,
Dave   AB7E




On 12/28/2014 1:06 PM, Rudy Bakalov wrote:
> So here's a bit more of my thinking:  my main goal is improved RX 
> performance so does current distribution, presumably a TX 
> consideration, really matter?  The loop is recognized as a low noise 
> antenna so increasing its size should only improve SNR. No?
>
> Then the other comparison I draw is to small RX loops, which also are 
> widely acknowledged as effective RX antennas. So what if we make the 
> small loop big. Really big. Full wavelength big. Is it still effective 
> as a RX antenna?
>
> Rudy N2WQ
>
> Sent using a tiny keyboard.  Please excuse brevity, typos, or 
> inappropriate autocorrect.
>
>
> On Dec 28, 2014, at 1:21 PM, David Gilbert <xdavid at cis-broadband.com 
> <mailto:xdavid at cis-broadband.com>> wrote:
>
>>
>> In general, there is nothing beneficial about simply having more wire 
>> as long as what you already have is electrically efficient.  Yes, 
>> there are some configurations where "more wire" can help ... a loop 
>> high enough off the ground has a db or so gain over a dipole at the 
>> same average height, a curtain array has gain, a dipole extended in 
>> length with capacitance has a bit of gain, etc ... but in your case 
>> your additional wire simply moves more of the current distribution 
>> closer to ground, thereby raising your takeoff/receive angle.  It 
>> might be a bit quieter, but not necessarily in a good way ... in my 
>> opinion.
>>
>> If you want to hear better toward Europe, maybe you could try running 
>> a rope westward off your tower to an anchor point as far from the 
>> tower as you can get it, and then hang a reflector wire off that rope 
>> about 100 feet away from the tower.  The reflector would be pretty 
>> close to the ground, but might ... might ... offer some benefit.   
>> Slight improvement in forward gain, modest improvement in rearward 
>> rejection, possibly decent sideward rejection, very slight lowering 
>> of takeoff angle.  If probably would alter your current tuning, but 
>> might be worth a try.
>>
>> 73,
>> Dave   AB7E
>>
>>
>>
>> On 12/28/2014 7:35 AM, Rudy Bakalov wrote:
>>> I actually do have the exact same setup- an inv-V at roughly 90' 
>>> with both ends sloping a couple of hundred feet away from the tower. 
>>> The apex is more likely ~120 degrees than 90. It works really well 
>>> (skimmer data shows actual performance much better than 
>>> theoretical), but I was wondering if I can improve on it, 
>>> particularly on receive where I feel I don't hear well enough. I 
>>> guess my question is "if a piece of wire of certain length works 
>>> really well, what happens if I double the length of the wire".
>>>
>>> Rudy N2WQ
>>> ------------------------------------------------------------------------
>>>
>>> I'm curious what you desire to achieve from such an antenna.  It's not
>>> likely to be an improvement over what you already have.
>>>
>>> A few years ago, before I was able to put up my current tower and yagis,
>>> I suspended a 40m diamond-shaped loop from a 55 foot tall pole ...
>>> sloped toward Europe from here in Arizona and fed at the bottom apex.
>>> It worked OK, mostly because I live on a steep hillside facing Europe,
>>> but in my opinion it didn't offer anything special other than it was
>>> also possible to use it on other bands with a decent tuner.  Keep in
>>> mind that unless you feed it at one side for vertical polarization that
>>> half your current distribution is going to be pretty close to the ground.
>>>
>>> In my opinion, you'd be better off stringing an Inverted-V from the top
>>> of your tower if you want something horizontally polarized to complement
>>> your existing verticals.
>>>
>>> 73,
>>> Dave   AB7E
>>