Increase Sensitivity / Foxhunt Performance

Hey all. Just kinda asking for opinions here as I am still not much of an expert on rf chains. What I originally had was a krakensdr with the standard coax they ship you with the antenna set, as well as the antenna set of course. The issue is the kraken would sometimes struggle to pick up a strong enough signal to start navigation to the fox. Once it’s established, we’re good, but getting those initial bearings is tough sometimes. I’m operating on VHF frequencies for this fox but I’d like this to be capable for UHF too.

The scale of my local foxhunt is like 17 miles x 15 miles area roughly and the terrain is pretty flat/sprawling suburban with only a few high spots to pick up a signal. I happen to know the transmitters range from 300mw to 700mw with HT whips. The city core is also full of RF noise which doesn’t make it easier.

In an effort to improve this, I have gotten 5 of the RTL-SDR LNAs and put them right on top of the krakensdr. I know the rf thing to do is put them near the antennas to overcome coax losses, but I don’t much like the idea of those being outside the car. When I added the LNAs, I had to reduce the gain from ~18db to 0db of gain to keep the noise floor low enough. There were several spikes around the band (full width 2.4mhz) that admittedly did improve when I left town. I guess I have 3 questions:

  • Is the LNA solution better than using internal gain on the kraken? Will they mess with phase coherence more than about 5 degrees? I can live with inaccuracy lower than that if it means I pick up the trail sooner.

  • Would it be worth it to replace the included coax with something like LMR-195UF? I have some and the ability to make phase coherent ones by VNA, but if it’s going to be minimal improvement I’m not sure I should bother.

  • I know there can be issues using bandpass filters because the minute differences in construction can throw off phase coherence, so I haven’t gotten any of those yet, instead I was wondering if decimation will produce any discernible effect? The effects I would expect are blocking out adjacent signals, therefore improving the noise floor, and by virtue of that, allowing the gain to be increased without fear of overdriving anything. In this case, does decimation operate exactly like a filter, and if not, how are they different?

I realize that’s a comprehensive set of issues, and I appreciate anything anyone has to say on this.

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Yes adding LNAs can mess with phase coherence, but it should not be a huge difference if they’re all the same type. How much difference is hard to say because it could vary with silicon.

An LNA will help you most on the higher frequencies (400 MHz+) which is where the NF improvement will be most effective, and where the gain will make the most effect at mitigating coax loss.

If your fox’s are at lower freqs, getting thicker lower loss cable might be a better idea than using LNAs.

Decimation is not the same as using external filtering. Decimation can’t fix if the tuner is being overloaded by something strong. It can only help to isolate an already well received signal and perhaps reduce some quantization noise.

Another thing to consider is getting whip antennas sized or tuned correctly for your fox frequency, or at least taking a VNA to the telescopic antennas and determining the best length for your fox freq.

Right, so the most optimal situation I’m hearing is get antennas tuned for vhf (or take a vna to the telescopic, at the moment I’m just pulling them out to full extension), then bandpass filter, then LNA (or maybe not, I just added it because of the filter), then low loss coax to kraken, which itself should run as high a gain as it can without getting swamped by other transmitters.

Do you know what the typical loss figures is for the stuff included in the antenna kits? I thought it was rg174, and if it is, online calculators show a loss of about 0.6db

The cables are LMR100 (not the official LMR brand though, just a clone of LMR100). The loss is a bit better than RG174.

But yeah if you’re at VHF, there isn’t much to gain from better cables and an LNA. The biggest contributor will be tuned antennas.

Based on A LOT of fox hunting experience with the KerberosSDR (our radio club has done multiple foxes every weekend continuously for several years now) I generally set gain to 25.4 if I can’t hear the fox on my mobile radio. Then I find the highest place possible in the general direction to get an initial listen. Once I can hear the fox on my mobile radio and get a bearing from the Kerberos, I’ll recalibrate at 15.7 db to help cut down on false positives from reflections. I think the Kraken software will work slightly differently, hopefully that is an improvement. My point being that you probably don’t need any additional amplification.

I’m to the point where I might just make a huge fixed wing UAV and mount my kraken to that to get the locations… It’s getting a bit ridiculous where some of the foxes are, in valleys and whatnot where you have to essentially just drive on the right road by chance.

On the fox hunting thing - I’ve been invited to participate in one this coming weekend.

Frequency will be 146.535. I plug that frequency into the spreadsheet and it tells me that for a spacing multiplier of 0.2, I need to have an inter-element spacing of 40.95cm.

So if I am using the antenna spacer template (had a buddy 3d print it for me), I need to figure out how to get the antennas into what would be a 400mm spacer hole?

Am I understanding spacing correctly?

If so, I might need to find a bigger car!

The interelement spacing is the distance from one antenna to the next. Each antenna will be 41cm away from the next. Does that make sense?

No, not really. Not taking into consideration the use of the antenna placement arms.

That has me very confused.

Each antenna should be 34cm from the dead center of the array. That would give you 39.96939716 cm between the antenna elements (interelement spacing) if you were to measure from the end of each antenna to the next one and so on… in a circle around the pentagram shape.

Not across, or toward the center, just measure between antenna 0 and 1, then 1 and 2, then 2 and 3, then 3 and 4, and 4 back to 0.

It’s easier to just set the distance from the center (34cm) and if you do it right, the 39.969cm will fall into place provided the angles for each arm are correct.

The angles for the 5 arms should be even multiples of 72 starting at 0 until you get to 360 (or back to 0).

Antenna 0: 0 degrees
Antenna 1: 72 degrees
Antenna 2: 144 degrees
Antenna 3: 216 degrees
Antenna 4: 288 degrees

I hope this helps.

FYI: I chose 34cm from CENTER because it got the inter-element spacing close to 40cm to fit your needs.You really just need to have all the elements the same distance from center and far enough out to give you decent resolution on the frequency in question. The interelement spacing will fall in line if you do it that way.

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Thank you for the explanation. Y’all have to forgive me as I’m very much a product of the 90’s American school system!

So if I lay out the arms from the center piece they are at 50, 100, 150, 200mm. According to my understanding from the documentation.

If I’m doing my GoogleFu correctly, I should be interested in the 350mm spacer hole, correct?

That’s what it sounds like. 350mm = 35cm, which would give you 41.14496766cm of interelement spacing as long as the arms are at the correct angles. Antenna 0 should be toward the front or direction you are going.

It would give you about 20 degrees of resolution (0.2 wavelength) at that distance, and you’d put 0.35 meter in the box in the web setup for the UCA.

The fox hunt was a bust for me this morning. I did find out where he was later in the day and i was probably 5-7 miles away.

The Kraken didn’t pick anything up, neither did my HT with squelch completely off. Had I kept going the way I was going, I would have eventually found him. But after riding around for almost 3 hours, the bladder was full and I was ready for a break.

I want to go out and “hunt” for a known source. So I’m going to try looking for the club’s repeater.

One thing I didn’t do, was key up my HT to ask questions on the repeater. I’m really freaked out about blowing out the receivers in the Kraken.

Here’s a shot of how I laid out the antenna array -

Hm yeah unfortunately not much can be done if the signal isn’t strong enough. If the terrain permits, I would start searching from an elevated position, on a hill or something. Worst case you’ll have to start with a Yagi type direction finder, get a heading, and use Kraken to pinpoint once you’re a little closer.

Can you double check the imgur link? It’s not working on my end.

It’s working for me. Are you getting an error or something?

I’m thinking next time, I look for his input frequency to the repeater to get a general bearing. Heh.

I think my Arrow antenna would have been helpful in this. But it’s a bit large for me to handle in the car and being alone.

Working now, must have just been a temporary glitch. Antenna layout looks good to me.

If you need to key up to ask a question, I would just power down the Kraken and Pi4, ask the question, then power up again. When it’s off there won’t be damage.

I live in a large city with a big amount of energy in the broadcast FM band (88 to 108 MHz), and my RTL SDR did not worked well. Then I bought the blog’s official FM notch filter. My reception got a huge improvement! Now I can set the RF gain to the maximum with no worsening of the noise. Before that, I suppose the RTL was saturated with the strong out of band unwanted signals.

Then I purchased the Kerberos SDR (the first version ever) and four filters as well. It works well with the filters, they seem not to degradate phase coherence in excess. I always set RF gain of the Kerberos SDR to the maximum. I am unsure if this is optimal, and interested in advice, but at least with the four FM notch filters the noise foor improves when the RF gain is maximum.

Perhaps this is what you need? I would suggest to buy only one of them at first, and evaluate the improvement with the normal RTL SDR. Then you could decide if you want three or four more ones.

If I correctly understood, you hunt the fox in the field, but not far away from a city. Perhaps you have the same problem? Is there a local medium wave, or more probably, FM broadcaster?

Please note that LNAs make the signal stronger, but do the same or even worse with noise. If you live in an environment with strong out of band emissions, like me, inexpensive receivers such as RTL SDR will probably behave worse with a large RF gain. I suggest you to focus in signal to noise ratio, not just signal level. It was one of the first advices older VHF ham radio operators gave to me when I was a begginer. Adding more and more gain could be the just the opposite of what you need.

I just received my SDR today, and have a foxhunt next weekend. I had pre-ordered some band pass filters. I hope that, along with pretuning the antennas will work out well. I’ll probably do a test run or 2 over the weekend to find “known” transmitters, and then see how next weekend goes. I’ll still havea HT and Yagi with me for backup. :slight_smile: