Meshtastic / Meshcore – Bandpass Filter – Comparison

For this post I will be presenting some performance data that I was able to measure for a number of a different 900Mhz ISM band filters as they relate to Meshtastic and MeshCore. The methods uses for collecting these measurements is covered in my previous posts.

Disclaimer: Some of the filters used for this post were provided to me from a local meshie to evaluate. Between the local meshie and myself we indidvidually purchased these filters with our own funding and neither of us are related to the sellers of these filters.

For the performance data for each filter I have included two plots, one plot illustrating the filter center frequency and cutoff frequencies, the second plot includes markers indicating the performance on the Meshtastic, Meshcore frequencies and a reference marker indicating the center frequency. These plots are all shown using an identical scale to allow the reader to compare the plots side by side if desired.

After going through the raw data, Ill provide a summary and my personal recommendations.

Aliexpress – Filter #1

Cost: $8.63

Link: https://a.aliexpress.com/_mMrQpHN

This filter features a bit of a sizable circuit board with mounting holes and dual SMA connectors. The filter itself is located within a shielded tin.

Figure 1c. Aliexpress #1 – Center and cut off frequencies

Figure 1d. Aliexpress #1 – Meshtastic/Meshcore and center frequencies

While this filter is clearly a band pass filter for the 900Mhz, band one dislike that I had with this particular filter is that the center frequency was found to be around 916Mhz which is above the standard Meshtastic and Meshcore frequencies. Both the MC and MT frequencies had higher insertion loss than some of the other filters tested.

Aliexpress – Filter #2

Cost: $8.04

Link:http://a.aliexpress.com/_mLdoUbz

This filter features a small board, given its size there was no room for fixed mounting holes. Like many of the other filters this one uses SMA connectors. It would have been handy if the SMA receptacle was a box mount water resistant style so that it had the option for directly mounting to an enclosure. This filter provides no shielding for the filter stage.

Figure 2c. Aliexpress #2 – Center and cut off frequencies

Figure 2d. Aliexpress #2 – Meshtastic/Meshcore and center frequencies

Given it being one of the more inexpensive filters, this filter had approximately -1dB of loss for both MT and MC and a slightly wider bandwidth when compared to the Aliexpress #1 filter above.

Aliexpress – Filter #3

Cost: $18.71

Link: https://a.aliexpress.com/_mr64KUX

This filter, similar to the previous filters evaluated, includes SMA connectors but unlike the previous options it has a cylindrical piece that helps to protect and shield the circuit board inside the filter. I should note this filter does not include any O rings or gaskets for protecting the insides against the elements meaning this filter should still go inside a protected enclosure or should be sealed appropriately.

Figure 3c. Aliexpress #3 – Center and cut off frequencies

Figure 3d. Aliexpress #3 – Meshtastic/Meshcore and center frequencies

At the MT and MC frequencies this filter seemed to have similar performance compared to the Aliexpress #2 filter given its bandwidth and MT/MC insertion loss. I did not verify it but I suspect both of these filters may use the same filter chip.

WEHOOPER4

Cost: $25

Purchase: Contact WEHOPPER4 on Discord

The WEHOOPER4 filter was a fairly small size and include a set of mounting holes. At first I wasn’t sure how I felt about the board having UFL connectors on it as these tend to be delicate however from a trying this board out on a few nodes I found that the UFL connections helped to minimize the overall space required.

Figure 4c. WEHOOPER4 – Center and cut off frequencies

Figure 3d. WEHOOPER4 – Meshtastic/Meshcore and center frequencies

Out of all of the inexpensive filters tested, the shape of the WEHOOPER4 tended to be the smoothest filter with the pass band including two humps. The WEHOOPER had one of the largest pass band range and also had the highest insertion loss for MT and MC.

BayMesh BM3

Cost: $100

Purchase: Contact nullroutine on Discord

The BayMesh BM3 should not be compared too closely to the other filters evaluated here as it is a cavity filter that is designed to have a sharper cutoff frequency and because of this it has a physically larger size. The enclosure features mounting holes on the bottom and screws on the top for retuning the filter to different frequencies. SMA connectors are included on the enclosure for the RF connections.

Figure 5c. BayMesh BM3 – Center and cut off frequencies

Figure 5d. Bay Mesh BM3 – Meshtastic/Meshcore and center frequencies

Figure 5e. Bay Mesh BM3 – Center and cut off frequencies
Note: This chart has a modified frequency range

From comparing the measurements for the BayMesh BM3 you can see why this cavity filter is in a class of its own. It should be noted that this filter was specifically tuned for Meshtastic and that the Meshcore frequencies shown in the graphs could be tweaked with retuning the filter. When purchasing this filter I provided the frequency I was going to be using it at and the filter was delivered pretuned.

The insertion loss at the MT frequency was found to be a fairly low -1.07dB with a sharp bandwidth of 6.75Mhz which is significantly narrower than the other filters evaluated here meaning this filter can be helpful with mitigating in band interference issues.

While I have not evaluated them here its worth mentioning that BayMesh does offer additional filter options with wider filter bandwidth and lower insertion loss.

Summary

A table summarizing the characteristics of the filters evaluated here are shown below.

Table 1. Filter Summary

Selecting a filter for an application heavily depends on what you are trying to filter out. From these tests I have some basic recommendations.

For devices located in areas that are a high RF environment and likely have in-band interference choosing a cavity filter such as the Bay Mesh BM3 is going to be a good choice. Given the low bandwidth of this filter and its low insertion loss at the frequencies of interest make it an excellent choice.

For situations were a radio may not be seeing as high of an RF environment but a band pass filter is still wanted and cost is a factor, the Aliexpress #2 filter is what I would recommend. Most of the <$30 filters had similar pass band, bandwidths which only varied by a few Mhz. The insertion loss is a next significant criteria along with cost. These constraints are what leads me to recommend Aliexpress #2 filter.

There are any additional filters we should test feel free to reach out!

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Note: Permission has been expressly granted for this topic write up to be reposted on www.eastsidemesh.com