[Skimmertalk] [RBN-OPS] Recommendations for a good, small active antenna for the RBN

Sat Jun 8 16:41:27 EDT 2019

Hello Pete,

your concerns are justified but luckily the designer PA0RDT of the original mini-whip started about end 2017/beginning 2018 (IIRC) to implement a choke (RF isolating transformer) within the unit which resides normally inside the building near your receiver. That means, inside the housing you have a small switch which allows you to either pass the braid of the coaxial cable further to your receiver *OR* to block it by the mentioned RF isolating choke. This cool modification helps a lot but if you pay attention for proper installation of a mini-whip you won't need it. I googled and found a schematic so you will understand, look here:

http://www.472khz.org/media/pa0rdt-Mini-Whip-Manual.pdf

Keep in mind that in both cases -using a conducting mast or a non-conductin fiberglass mast- the braid of your coaxial cable acts as a conductor and thus as a path for the second pole of the antenna until it reaches end/ground. For example: you install a 4m fiberglass mast on the ground in your yard with the mini-whip installed on top. The coaxial cable running down the fiberglass mast is a conductor and the active element "sees" it. The best practice -although on the new PA0RDT miniwhip devices you have the RF isolatin transformer- is --> ground the braid of your coaxial cable as soon as it reaches ground on the shortest possible path. The PDF mentioned above shows a simple schematic in page 3/4. There you will see such a ground rod. Better to place it next to the mast, that's why I said on the shortest possible path.

An active antenna like this mini-whip is not just "plug-and-play" it needs a solid environment and proper setup. In all other cases you certainly will get disappointed and I guess that is the reason why so many hams had bad experience with it.

HTH, wish you good luck and looking forward to your new skimmer.

73
Saki, DD5XX

Gesendet: Samstag, 08. Juni 2019 um 14:29 Uhr
Von: "N4ZR" <n4zr at comcast.net>
An: "DD5XX (Saki)" <dd5xx at web.de>
Betreff: Re: [RBN-OPS] Recommendations for a good, small active antenna for the RBN

Thanks, Saki!  One thing that occurs to me is the possibility that the mini-whip gets a lot of its signal from common mode currents on the feedline.  Did you use common-mode chokes at one or both ends?
73, Pete N4ZR
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On 6/8/2019 3:41 AM, dd5xx at web.de[mailto:dd5xx at web.de] wrote:
Hello all,

the bad reputation of the mini-whip which red on the past posts here is due to the fact that an active antenna like the mini-whip needs understanding in proper operating. It's partly correct to mount the antenna as high as possible. The most important part is being far away from obstacles and surrounding environment but the most impact makes the height between the active element and mostly the receiver height. You need to know that the active part inside the mini-whip element on the top of the mast is one of the antenna pole and the other pole of the antenna is the ground. Now it depends on HOW you connected your mini-whip to your receiver. I will show two examples:

(a) the mini-whip is installed  on a 5m tall mast onto your roof and you have connected the mini-whip antenna with a coaxial-cable to your receiver which is located 15m beneath the mini-whip let's say e.g somewhere @2nd floor inside a building. The antenna itself in its entity is 10m height (!) this is the height difference of the mini-whip and the ground it sees which is your receiver. But this setup is totally failed for an active antenna setup, its way too high and far away from being optimal.

(b) the mini-whip is installed on a 2m mast right over the ground inside the large yard. Most of you now will say: "what the hell, only 2m over ground? are you kidding?" but --> this setup will certainly outperform setup (a) at the same location, provided a location as explained in the beginning.

With this height you can adjust the sweep spot on where your mini-whip will operate best and on how good it will perform as a multiband antenna. At 160m they will do a great job most of the time without a special height. But for those who say a mini-whip is bad at the upper bands, I suggest to reduce your height and give a try. You will be amazed. We did some comparisons 2 years ago with a 5-band Spiderbeam @15m AGL and the mini-whip was outstanding and showed nearly the same SNR as the spiderbeam even on 17m. You never will be able to have a mini-whip as strong on all bands, but if you configure it for 20m you have a good sweet spot.

Some time ago I posted some links, they fit nice to this topic and are crucial in my humble opinion.
 http://www.pa3fwm.nl/technotes/tn07.html[http://www.pa3fwm.nl/technotes/tn07.html][http://www.pa3fwm.nl/technotes/tn07.html[http://www.pa3fwm.nl/technotes/tn07.html]]https://www.pa3fwm.nl/technotes/tn09d.html[https://www.pa3fwm.nl/technotes/tn09d.html]

However, wish you good luck and hope you will find a suitable setup for your purpose.

73
Saki, DD5XX

Gesendet: Freitag, 07. Juni 2019 um 22:22 Uhr
Von: "SM0MDG" <bjorn at sm0mdg.com>[mailto:bjorn at sm0mdg.com]
An: RBN-OPS at groups.io[mailto:RBN-OPS at groups.io]
Cc: skimmertalk <skimmertalk at contesting.com>[mailto:skimmertalk at contesting.com]
Betreff: Re: [RBN-OPS] Recommendations for a good, small active antenna for the RBN

Hi Pete,

I have been using the mini-whip this season. I found it very good on Topband, decent on 80 meter then gradually weaker on higher bands. I’ve done some comparing with other nearby skimmers. Check my blog;

Antenna described here;http://blog.se0x.info/?p=3486[http://blog.se0x.info/?p=3486][http://blog.se0x.info/?p=3486[http://blog.se0x.info/?p=3486]]

Skimmer analysis;http://blog.se0x.info/?p=3549[http://blog.se0x.info/?p=3549][http://blog.se0x.info/?p=3549[http://blog.se0x.info/?p=3549]][http://blog.se0x.info/?p=3549[http://blog.se0x.info/?p=3549][http://blog.se0x.info/?p=3549[http://blog.se0x.info/?p=3549]]]

73 de Björn,
SM0MDG
SE0X

On 6 Jun 2019, at 03:30, Pete Smith <n4zr at comcast.net[mailto:n4zr at comcast.net]>[mailto:n4zr at comcast.net[mailto:n4zr at comcast.net]] wrote: 

I'm looking for ideas for an antenna that can be used with the new 16-bit Red Pitaya on the RBN, and, most importantly, can survive truly severe cold weather and wind at a polar location.  I had hoped that the PA0RDT "mini-whip" might do the trick, but so far I have not been able to get one to receive well at all, and have read recently on the web that a reputable expert says it really should not, from the standpoint of antenna theory.
So the question is, can anyone suggest an alternative?  It needs to be operable within 2-3 meters above the roof of a metal building, and absolutely bulletproof.
Thanks in advance. 