What are best specifications for cavity filters for 1420 MHz Hydrogen Line work?
Andrew Thornett
For 1420 MHz (the H I line at 1420.4058 MHz), the “best” cavity filter depends on where you place it in the chain (pre-LNA vs post-LNA) and what you need to reject. Below are practical, astronomy-focused specs that work very well in the field.
Recommended target specs (bandpass cavity filter, 50 Ω)
-
Center frequency (f₀): 1420.406 MHz (tunable ±2–3 MHz via screws)
-
Passband bandwidth (-3 dB):
-
10 MHz (good all-round preselector for spectral work)
-
20 MHz (if you want looser passband and lower insertion loss)
-
Rationale: even ±300 km/s Doppler shift is only ≈±1.42 MHz, so 10 MHz comfortably covers Galactic velocities while giving good nearby-band rejection.
-
Topology / poles: 3- to 4-pole Chebyshev (0.1 dB ripple) or quasi-elliptic (if you want steeper skirts).
-
3-pole is a great low-loss pre-LNA option; 4-pole buys you much better skirts if you have tough RFI.
-
Insertion loss:
-
≤ 0.5–0.8 dB (3-pole), ≤ 1.0 dB (4-pole) at f₀ (critical if used pre-LNA).
-
Return loss: ≥ 15 dB across passband (≥ 20 dB ideal).
-
Out-of-band rejection (typical goals):
-
≥ 40 dB at ±25 MHz from f₀
-
≥ 60 dB at ±40 MHz (helps with services near 1.45 GHz)
-
≥ 80 dB by ±100 MHz (keeps strong out-of-band energy out of the front-end/LNA).
-
Group delay variation (over passband): ≤ 150 ns (10 MHz BW) to preserve spectral line shape.
-
Power handling: Not critical for RX; ≥ +20 dBm is ample.
-
Temperature stability: Drift ≤ ±5 ppm/°C (aim for silver-plated copper cavities, low-TCR hardware); include lock nuts after tuning.
-
Connectors: N-type female (50 Ω) or SMA if fully lab-grade and mechanically protected.
-
Mechanical: Air-dielectric coaxial cavities (silver-plated copper or brass), unloaded Q ≈ 3000–8000. Use rigid coupling loops/pins with adjustable iris/screws for inter-resonator coupling.
-
Shielding: Seam-welded or well-gasketed enclosure; internal partitions between resonators to minimize parasitics.
Two proven “good” configurations
A) Pre-LNA preselector (minimal noise hit)
-
Use case: Right after the antenna, before the first LNA.
-
Spec: 3-pole Chebyshev, 10 MHz BW, IL ≤ 0.6 dB, RL ≥ 18 dB, rejection ≥ 55 dB @ ±40 MHz.
-
Why: Keeps strong off-band energy out of the LNA while adding very little to the system noise temperature.
B) Post-LNA cleanup (tough RFI)
-
Use case: Immediately after a low-NF LNA.
-
Spec: 4-pole quasi-elliptic, 10–15 MHz BW, IL ≤ 1 dB, RL ≥ 18 dB, deep skirts (≥ 70 dB by ±40–50 MHz).
-
Why: With SNR already boosted by the LNA, you can afford a bit more loss to get aggressive rejection of nearby services (e.g., around 1450 MHz).
Practical notes for 1420 MHz astronomy
-
Bandwidth choice:
-
10 MHz passband is a sweet spot: wide enough for all Galactic H I velocities and Earth/Sun motions, yet narrow enough to meaningfully preselect.
-
If you only do continuum imaging or need maximum sensitivity pre-LNA, a 20 MHz passband can shave ~0.2–0.3 dB insertion loss.
-
Order (poles):
-
3-pole for low IL before LNA.
-
4-pole when RFI is nasty or when placed after LNA.
-
Materials & finish: Silver-plate the resonator rods and cavity walls; use clean, tight RF gaskets; avoid ferrous hardware inside the RF volume.
-
Tuning workflow:
-
Individually peak each cavity at 1420.406 MHz (weak coupling).
-
Set inter-cavity coupling to hit the target BW.
-
Iterate source/load coupling for RL and IL.
-
Finalize with warm-up to operating temperature and re-touch.
-
System placement tips:
-
If very strong out-of-band signals are present, put a low-loss 3-pole cavity before the LNA.
-
Add a sharper 4-pole after the LNA (or before the SDR) to protect the ADC and clean the band further.
Example spec sheet you can hand to a fabricator
-
Type: Coaxial cavity bandpass filter, 50 Ω
-
Center: 1420.406 MHz (mechanically tunable ±3 MHz)
-
Order: 3-pole Chebyshev (0.1 dB ripple)
-
BW (-3 dB): 10.0 ± 0.5 MHz
-
Insertion Loss: ≤ 0.6 dB @ f₀; ≤ 0.9 dB over passband
-
Return Loss: ≥ 18 dB over passband
-
Rejection: ≥ 55 dB at 1380 MHz and 1460 MHz; ≥ 75 dB at ≤ 1320 MHz and ≥ 1520 MHz
-
Group Delay Ripple: ≤ 120 ns pk-pk over passband
-
Power: ≥ +20 dBm CW (receive-only)
-
Temperature: Drift ≤ ±5 ppm/°C; locking hardware provided
-
Connectors: N-female both ends (SMA optional)
-
Finish: Silver-plated internal RF surfaces; black anodized exterior
-
Size: Typical for 3-resonator 1.4 GHz unit (fabricator to optimize for Q and IL).
External-Q Targets and VNA Tuning Plan:
Low-pass prototype (0.1 dB ripple, N=3)
-
(g_0=1), (g_1=1.5963), (g_2=1.0967), (g_3=1.5963), (g_4=1)
For a bandpass with fractional bandwidth FBW = BW / f₀ (use (f₀=1420.406) MHz):
-
Inter-resonator coupling
( M_{12}=M_{23}=\dfrac{\text{FBW}}{\sqrt{g_1 g_2}} ) -
External Q (source & load)
( Q_{e1}=Q_{e3}=\dfrac{g_0 g_1}{\text{FBW}}=\dfrac{g_1}{\text{FBW}} )
Ready-to-use targets
| 3-dB BW (MHz) | FBW | (M_{12}=M_{23}) | (Q_{e1}=Q_{e3}) |
|---|---|---|---|
| 8 | 0.005632 | 0.004257 | 283.42 |
| 10 | 0.007040 | 0.005321 | 226.74 |
| 12 | 0.008448 | 0.006385 | 188.95 |
| 15 | 0.010560 | 0.007981 | 151.16 |
| 20 | 0.014080 | 0.010642 | 113.37 |
For the “sweet-spot” 10 MHz passband: M12=M23 ≈ 0.00532, Qe1=Qe3 ≈ 227.
How to hit these numbers on the bench (VNA method)
A) Calibrate & prep
-
VNA cal: Full 2-port cal with your actual cables/adaptors (42 dB dyn. range or better if you can).
-
Set span: Centre 1420.406 MHz, span ~100 MHz for early coarse work; narrow later.
-
Couplers/loops: Start with weak source/load coupling (aim high Q) and minimal inter-cavity iris/loop coupling.
B) Single-resonator tune (each cavity alone)
-
Port-1 reflect (S11) with cavity as a 1-port resonator (loosely coupled).
-
Peak the tuning screw/slug to 1420.406 MHz. Record unloaded-Q trend (marker bandwidth method).
-
Repeat for all three cavities.
C) Pairwise coupling to set M
Use the split-frequency method on two coupled, identical resonators:
-
Connect two cavities only (no 3rd), with very loose source/load coupling.
-
Tune both to the same f₀ (as above).
-
Slowly increase inter-cavity coupling until the single resonance splits into two at (f_1) and (f_2).
-
Compute ( K ) (aka (M) for narrowband TEM cavities):
[
K ;\approx; \frac{f_2^2 - f_1^2}{f_2^2 + f_1^2}
]
Adjust spacing/iris until (K \approx M_{12}) target from the table. -
Repeat for cavities 2–3 to hit (M_{23}).
Tip: For small FBW filters like 0.7 %, the (f^2) form is more accurate than ((f_2 - f_1)/(f_2 + f_1)).
D) Set external Q to target (Q_e)
With only cavity-1 connected to Port-1 (very weak coupling), measure S11 around f₀:
-
Use the phase-slope or −3 dB bandwidth method for a singly loaded resonator.
-
Practical shortcut:
[
Q_e ;\approx; \frac{f_0}{\text{BW}_{\text{3 dB,,S11 dip}}}
]
(with coupling light enough that the dip is shallow and dominated by coupling, not resistive loss). -
Adjust the source coupling loop/iris until (Q_{e1}) matches the table (e.g., ≈227 for 10 MHz BW).
-
Repeat at the output for (Q_{e3}).
E) Assemble all 3, final tune on S-parameters
-
Cascade all three. Start with the previously set M’s and Qe’s.
-
Observe S21: You should see ~Chebyshev ripple (≈0.1 dB) across the passband and the correct −3 dB bandwidth.
-
Iterate very small tweaks:
-
Inter-cavity coupling sets total BW and skirt steepness.
-
Source/load coupling trades passband ripple vs return loss.
-
Re-touch each resonator screw to keep centre at 1420.406 MHz after mechanical changes.
-
Targets to confirm:
-
IL ≤ 0.6–0.8 dB (3-pole)
-
RL ≥ 18–20 dB across passband
-
BW within ±0.5 MHz of goal
-
Ripple ~0.1 dB (Chebyshev signature)
-
Skirts meet your earlier rejection goals (e.g., ≥55 dB @ ±40 MHz for the pre-LNA unit).
Turning M and Qe into physical settings
-
Inter-resonator M grows with larger iris aperture, closer loop spacing, or stronger probe alignment; reduce M by increasing gap or rotating loops to reduce mutual coupling.
-
External Qe increases (weaker coupling) as you pull the input/output loop away from the E-field region, shrink loop area, or rotate it to reduce magnetic coupling.
-
Make adjustments symmetrically to keep passband flatness; if ripple is higher near one edge, that side’s Qe/M is slightly off.
Quick sanity checks
-
If passband is too narrow: (M)s are too small or (Q_e)s too high (too weak).
-
If passband is too wide: (M)s are too large or (Q_e)s too low (over-coupled).
-
If deep notch in passband: one resonator off-frequency; re-peak each cavity with the others temporarily de-coupled.
Alex P
Marcus D. Leech
Pre LNA is a poor choice as any Passive Loss adds 1:1 to total Noise Figure
But there are situations where you don't have a choice, where you have strong out-of-band signals driving your LNA into de-sense territory. We had that situation at the
observatory, due to a cell site about 1km away from the dish. We opted to have Bert, PE1RKI make us a pair of low-loss, wide-band (200MHz at the 3dB points) pre-filter
that allowed us to operate, these filters have about 0.1dB loss -- about the same as a connector.
Once you have a decent amount of low-noise gain, then cheaper filters like SAW or dielectric filters are perfectly adequate. This is the architecture of the SawBird+ H1 LNAs--
MMIC-LNA--->SAW-FILTER--->MMIC-LNA. In most circumstances, that's enough. In some situations, with closer-in annoyances that might drive the *receiver* front-end
into non-linearity, you might need another filter in front of the receiver, of more-modest bandwidth. The bandwidth of the SAWBIRD LNAs is about 65MHz, but that might
be a bit too "sloppy" in some situations. The radio astronomy allocation runs from 1400-1427MHz, although I'm unaware of any commercial SAW filer that covers that
range exactly....
Two proven “good” configurations A) Pre-LNA preselector (minimal noise hit)
Use case: Right after the antenna, before the first LNA.
Spec: 3-pole Chebyshev, 10 MHz BW, IL ≤ 0.6 dB, RL ≥ 18 dB, rejection ≥ 55 dB @ ±40 MHz.
Why: Keeps strong off-band energy out of the LNA while adding very little to the system noise temperature.
--
--
You received this message because you are subscribed to the Google
Groups "Society of Amateur Radio Astronomers" group.
To post to this group, send email to sara...@googlegroups.com
To unsubscribe from this group, send email to
sara-list-...@googlegroups.com
For more options, visit this group at
http://groups.google.com/group/sara-list?hl=en
---
You received this message because you are subscribed to the Google Groups "Society of Amateur Radio Astronomers" group.
To unsubscribe from this group and stop receiving emails from it, send an email to sara-list+...@googlegroups.com.
To view this discussion visit https://groups.google.com/d/msgid/sara-list/8a64bfd1-b615-4b2b-b0a4-0c21e7b6c4cdn%40googlegroups.com.
b alex pettit jr
I have a 3 stage cavity filter built by Tom Henderson ( formerly builder of RAS electronics )
andrew....@googlemail.com
The WTMicrowave filter below is pretty close - https://www.wtmicrowave.com/en/product/WT-A9347-Q06.html
1400-1420MHz Cavity Band Pass Filter:
- P/N： WT-A9347-Q06
- Center Frequency：1410MHz
- Pass Band Frequency: 1400-1420MHz
I have one – cost me £130.
Andy
--
--
You received this message because you are subscribed to the Google
Groups "Society of Amateur Radio Astronomers" group.
To post to this group, send email to sara...@googlegroups.com
To unsubscribe from this group, send email to
sara-list-...@googlegroups.com
For more options, visit this group at
http://groups.google.com/group/sara-list?hl=en
---
You received this message because you are subscribed to the Google Groups "Society of Amateur Radio Astronomers" group.
To unsubscribe from this group and stop receiving emails from it, send an email to sara-list+...@googlegroups.com.
To view this discussion visit https://groups.google.com/d/msgid/sara-list/1695855824.1665325.1761928291866%40mail.yahoo.com.