Dipole Vs loop

FM antenna discussion
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Patrick73
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Dipole Vs loop

Post by Patrick73 » 07 Apr 2015 11:51

Yesterday, I built a 'quick and dirty' Quad loop (single element) as described here by John : http://skegnessdx.blogspot.fr/2014/12/t ... tenna.html

This kind of antenna is supposed to provide up to 2dB gain over the dipole. I was curious to check reception on weak semi-local stations (abt 200 km from home) to confirm there is an improvment. But ... this dis not happen :-( On the contrary, against all expectations I found the dipole to offer a slightly better reception.

The only explanation could be I used 50 ohm coaxial for the downlead, instead of 75 ohms.
The receiving element itslef was made of (insulated) copper (1.5mm diam).

Could this explain its desappointing performance ?

Next, I plan to build a rectangle loop, providing 1.5dB more compared to the square and / or 50 ohms impedance, according to http://pa0fri.home.xs4all.nl/Ant/Quad/quadeng.htm
Patrick
Main interests : NDB, MW & FM DX'ing
REU Editor http://www.classaxe.com/dx/ndb/reu

satnipper
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Location: Western Europe

Re: Dipole Vs loop

Post by satnipper » 08 Apr 2015 22:43

Horizontal or Vertical? Maybe nearby objects which enhance or degrade reception. In any case I would challenge you to hear any improvement of 0.5 dBd through audio alone.

Stuart

Peter Körner
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Joined: 21 Oct 2014 10:05

Re: Dipole Vs loop

Post by Peter Körner » 09 Apr 2015 12:07

>This kind of antenna is supposed to provide +1/2dB gain over the dipole.

Well, this is what can be labelled as raw gain, but net gain, taking mismatch losses into account, is something else. The SWR bandwidth of quad loops is rather limited due to the high variation of the reactive impedance part.
For FM you are likely to cover only around 5 MHz with reasonable impedance match and low mismatch losses…


>On the contrary, against all expectations I found the dipole to offer a slightly better reception.

I’m not surprised.


>The only explanation could be I used 50 ohm coaxial for the downlead, instead of 75 ohms.

That mismatch is negligible compared to what is caused by the mismatch outside the narrow SWR bandwidth. The coax should be connected using a balun (could be a couple of good ferrite beads). E.g. see http://ham-radio.com/k6sti/balun.htm


>In any case I would challenge you to hear any improvement of 0.5 dBd through audio alone.

I agree.


Peter

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Patrick73
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Re: Dipole Vs loop

Post by Patrick73 » 09 Apr 2015 21:13

Thanks you both for the comments.
The Quad was Horizontal (feedline at the bottom).

<< In any case I would challenge you to hear any improvement of 0.5 dBd through audio alone >>
You're absolutly right, of course ! What I meant is that I expected (from what I read) at least similar performance but not the dipole to provide better reception. My english is a bit limited and it's sometimes a bit tough to express exactly what I intend to !

Anyway, it was a fun test (using the "deaf" ELAD barefoot ...)
Patrick
Main interests : NDB, MW & FM DX'ing
REU Editor http://www.classaxe.com/dx/ndb/reu

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zach_m
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Re: Dipole Vs loop

Post by zach_m » 01 Sep 2017 09:46

I build FM Quad loop not so long ago and it perform perfeclty. I used 75-ohm low loss coax (RG-6/U) for the loop and downlead. It pull out 94.1 BBC R4 from Tacolneston (126km) in Stereo almost every time I tune in. It has more gain than my folded diopole and in some cases outperform the dipole.

Here is the photo of the loop: https://c1.staticflickr.com/5/4387/3670 ... f140_o.jpg

Make sure you match the coax impedance, or like suggested by Peter Korner above, use balun.

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