Max Frequency Selection And Iron Falsing

[Lodge Scent]

I was reminded today why I got away from using Deep High Conductor on my D2. When I first got my D2 a few years back, I used Deep HC with some success but the iron falsing was bothersome. I had forgotten all that and decided to try it again today at one of my old colonial era sites to see if I could sniff out a deep silver or copper. But I was quickly reminded why I had not used Deep HC in a few years when the D2 started falsing over the crusty old nails and bean pot fragments. Out of curiosity I bumped the Max Frequency from the stock 14 kHz in Deep HC up to 24 kHz. I expected there would be some improvement but was surprised at just how much the falsing abated when going from 14 to 24 kHz.

I’ve never understood the relationship between frequency selection and iron falsing. Why does one end of the frequency range false more than the other end of the frequency range. Curious minds want to know. 😊

Thanks,

Lodge

[Chase Goldman]

This post from Geotech with an excerpt from his latest book on detector design, demonstrates the wide R - X phase angle response variation of a ferrous, flat steel target.  But other notorious ferrous "falsers" such as nail heads, annealed (burnt) square nails, and bent nails, plot similarly.

This shows that if you limit the frequency spectrum of a multifrequency detector (i.e., limit the max FMF cutoff frequency to 14 khz in the case of the D2) to favor high conductive targets, you lose a lot of higher frequency data that can help the detector better ID difficult ferrous masquerading as non-ferrous.  Therefore, you can be susceptible to more frequent ferrous falsing.  I like relic mode at 24 khz FMF as a decent middle ground to trade depth against falsing.  It also "lights up" mid-conductive targets better than 14 khz and below, which comprise the bulk of relic targets. 

From a methodology standpoint, if I still suspect ferrous falsing, I like to "interrogate" my targets with higher and lower max FMF variants of the same program or a different program with a different default FMF. 

The fact that you can quickly shift between custom programs on the D2 using the "+" and "-" keys facilitates quick target interrogation to make a fairly rapid dig decision.

This also supports the notion that it pays to re-survey a productive site with a different MF profile (or a different MF detector) to see what you might have missed.

Utilizing built-in filters such as Bottlecap Reject and Silencer can help too, but can also be subject to other tradeoffs.  I've found BC reject to be very effective in how it's implemented on D2.  You can still hear the BC reject chirp (so you know when you hit a BC instead of just pure silence, and I have seldom if ever found it to mask a keeper (on my peroidic sampling) or significantly impact depth.

Geotech's post also makes me now wonder if the D2 BC reject feature works less effectively or differently for different FMF Max settings.  Hmm...

 

[Lodge Scent]

Thanks Chase (and Carl) for the explanation. That makes sense to me. Funny, I've also settled on 24 kHz for most hunting for the reasons you give, but I set up my adjacent programs with different Reactivities rather than different frequencies. I will use some Silencer but I really haven't done much with BC. I'll have to play with that setting.

[F350Platinum]

The heavy hitters have weighed in LS, but I'll share that BC is useful both on the beach and relic hunting. I'm pretty convinced it's a minor to moderate filter. You've seen my posts, I get everything that's there and don't show any or many bottlecaps. Even got a small gold ring the last time I was at the beach. This is with BC at 5... 

 

Could be because the copper content in 10k is a smidge higher than 14k, but I can show you a few of those and a 22+k tiny gold ring I dug relic hunting with BC at 5. If your soil is mild like mine it shouldn't be a big issue to crank it up. 🙂

I run silencer at 0, and sometimes add max 40khz to relic for those on edge/tiny coins.

[Lodge Scent]

Thanks F350. Yeah the use of BC doesn't seem to affect the amount of goodies you put in your pouch!

[raziel900]

On 1/5/2025 at 7:13 PM, Chase Goldman said:

This post from Geotech with an excerpt from his latest book on detector design, demonstrates the wide R - X phase angle response variation of a ferrous, flat steel target.  But other notorious ferrous "falsers" such as nail heads, annealed (burnt) square nails, and bent nails, plot similarly.

This shows that if you limit the frequency spectrum of a multifrequency detector (i.e., limit the max FMF cutoff frequency to 14 khz in the case of the D2) to favor high conductive targets, you lose a lot of higher frequency data that can help the detector better ID difficult ferrous masquerading as non-ferrous.  Therefore, you can be susceptible to more frequent ferrous falsing.  I like relic mode at 24 khz FMF as a decent middle ground to trade depth against falsing.  It also "lights up" mid-conductive targets better than 14 khz and below, which comprise the bulk of relic targets. 

From a methodology standpoint, if I still suspect ferrous falsing, I like to "interrogate" my targets with higher and lower max FMF variants of the same program or a different program with a different default FMF. 

The fact that you can quickly shift between custom programs on the D2 using the "+" and "-" keys facilitates quick target interrogation to make a fairly rapid dig decision.

This also supports the notion that it pays to re-survey a productive site with a different MF profile (or a different MF detector) to see what you might have missed.

Utilizing built-in filters such as Bottlecap Reject and Silencer can help too, but can also be subject to other tradeoffs.  I've found BC reject to be very effective in how it's implemented on D2.  You can still hear the BC reject chirp (so you know when you hit a BC instead of just pure silence, and I have seldom if ever found it to mask a keeper (on my peroidic sampling) or significantly impact depth.

Geotech's post also makes me now wonder if the D2 BC reject feature works less effectively or differently for different FMF Max settings.  Hmm...

 

Is this notion valid for all multi-frequency metal detectors?

[Geotech]

24 minutes ago, raziel900 said:

Is this notion valid for all multi-frequency metal detectors?

The physics is valid, but how various models deal with it will likely differ.

[palzynski]

On 1/5/2025 at 3:30 AM, Lodge Scent said:

I was reminded today why I got away from using Deep High Conductor on my D2. When I first got my D2 a few years back, I used Deep HC with some success but the iron falsing was bothersome....

Same for me , one year ago I have tried Deep HC but I found it too unstable and noisy compared to the more "classical" modes like SENSITIVE FT for example

However a few weeks ago I had a discussion with a friend who is a MD dealer and he told me that he mostly uses DHC so I decided to retry it .

Now I use DHC quite often and I like it . Yes DHC audio is noisy compared to other modes , but the it goes definitely deep . It is a very good mode to boost depth on targets like medium-big coins ..

For info I have slightly changed the factory DHC settings  : number of tones 5 (instead of 2 ) ,  audio HIGH SQUARE ( instead of PWM ). I wont change the max frequency as I prefer to change a minimum of settings to keep it simple..

At the end I only use 2 modes on my D2 :

- Tekkna ( based on SENSITIVE FT )

- Deep HC

[Lodge Scent]

Tekkna is a favorite of mine as well Palzynski. I run it in PWM and have it set up in 3 slots with different Reactivities for quick changes.